Document:

<PAGE>

                                EXHIBIT 10(20)
                 Seventh Amendment to Business Loan Agreement
  between Registrant and Bank of America, Texas, N.A., dated August 26, 1999
<PAGE>

                 SEVENTH AMENDMENT TO BUSINESS LOAN AGREEMENT
                          (RECEIVABLES AND INVENTORY)

     This SEVENTH AMENDMENT TO BUSINESS LOAN AGREEMENT (RECEIVABLES AND
INVENTORY) (this "Amendment") is executed and entered into on August 26, 1999,
                  ---------
by and between Bank of America, N.A. (successor by merger to Bank of America,
Texas, N.A., the "Bank") and Aztec Manufacturing Co. (the "Borrower").
                  ----                                     --------

                                   RECITALS:

     A.  The Borrower and the Bank are parties to that certain Business Loan
Agreement (Receivables and Inventory) dated as of June 28, 1996 (as amended and
as the same may be further amended, renewed, extended, restated, or otherwise
modified from time to time, the "Loan Agreement") pursuant to which the Bank
                                 --------------
agreed to provide to the Borrower a secured revolving credit facility and a
secured term loan facility.

     B.  The indebtedness of the Borrower to the Bank pursuant to the Loan
Agreement is secured by liens on the Borrower's property as described in that
certain Security Agreement: Receivables, Inventory and Equipment (Borrower);

     C.  Each subsidiary of the Borrower is a Pledging Party and a Guarantor
under the terms of the Loan Agreement.

     D.  The Borrower has requested that the Bank provide additional loans to
the Borrower as set forth herein and amend certain provisions of the Loan
Agreement, and, subject to satisfaction of the conditions set forth herein, the
Bank is willing to make such additional loans available to the Borrower and
amend the Loan Agreement.

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto hereby agree as
follows:

                                   ARTICLE 1

                                  Definitions
                                  -----------

     Section 1.1  Definitions.  Unless otherwise defined in this Amendment, each
                  -----------
capitalized term used in this Amendment has the meaning given to such term in
the Loan Agreement (as amended by this Amendment).
<PAGE>

                                   ARTICLE 2

                                  Amendments
                                  ----------

     Section 2.1  Amendment to subsection 1.1(a) of the Loan Agreement.
                  ----------------------------------------------------
Effective as of the date hereof, subsection 1.1(a) of the Loan Agreement is
                                 -----------------
hereby amended and restated to read in its entirety as follows:

          (a) Twenty-Two Million Dollars ($22,000,000.00), or

     Section 2.2  Amendment to Section 1.4 of the Loan Agreement.  Effective as
                  ----------------------------------------------
of the date hereof, Section 1.4 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          1.4  "Pledging Party" means the Borrower and each subsidiary of the
                --------------
     Borrower who is party to a security agreement in favor of the Bank pledging
     to the Bank such subsidiary's accounts, inventory, equipment, and other
     related assets; provided, however, that Pledging Party shall also mean and
                     --------  -------
     include CGIT Westboro, Inc. ("CGIT") from and after the date of the
     acquisition of  CGIT by the Borrower, CGIT to execute and deliver to the
     Bank a guaranty, security agreement, financing statements and such other
     documents (including, without limitation, an officer's certificate as to
     CGIT's charter documents, resolutions and authorized signatures) required
     by the Bank to create and perfect a security interest in CGIT's accounts,
     inventory, equipment, and other related assets within ninety (90) days of
     the Borrower's acquisition of CGIT.

     Section 2.3  Amendment to Section 2.14 of the Loan Agreement.  Effective as
                  -----------------------------------------------
of the date hereof, Section 2.14 of the Loan Agreement is hereby amended and
                    ------------
restated to read in its entirety as follows:

          2.14  Libor Rate.  The Borrower may elect to have all or portions of
                ----------
     the principal balance of the loans bear interest at a rate equal to the
     lesser of (i) the Maximum Rate or (ii) the Libor Rate plus (i) the
                                                           ----
     Applicable Libor Margin (as determined as provided in clause (j) below)
     with respect to the Line of Credit or (ii) one and one-quarter percent
     (1.25%) with respect to the Term Loans (the "Eurodollar Rate"), subject to
                                                  ---------------
     the following requirements:

               (a) The interest period during which the Eurodollar Rate will be
          in effect will be (i) 1, 2, 3, or 6 months with respect to the Line of
          Credit or (ii) 1, 2, 3, 6, or 12 months with respect to the Term
          Loans.  The last day of the interest period will be determined by the
          Bank using the practices of the London interbank market.

               (b) Each Eurodollar Rate portion under a loan will be for an
          amount not less than Five Hundred Thousand Dollars ($500,000.00) or an
          integral multiple thereof.

               (c) The Borrower shall irrevocably request a Eurodollar Rate
          portion no later than 11:00 a.m. Dallas, Texas time two (2) banking
          days before the commencement of the interest period.

                                       2
<PAGE>

               (d) The "LIBOR Rate" means the interest rate determined by the
                        ----------
          following formula, rounded upward to the nearest 1/100 of one percent.
          (All amounts in the calculation will be determined by the Bank as of
          the first day of the interest period.)

                    LIBOR Rate =    London Rate
                                 ---------------------------
                                 (1.00 - Reserve Percentage)
          Where,

                    (i)  "London Rate" means the interest rate (rounded upward
                          -----------
               to the nearest 1/16th of one percent) at which the Bank's London
               Branch, London, Great Britain, would offer U.S. dollar deposits
               for the applicable interest period to other major banks in the
               London interbank market at approximately 11:00 a.m. London time
               two (2) banking days prior to the commencement of the interest
               period.

                    (ii) "Reserve Percentage" means the total of the maximum
                          ------------------
               reserve percentages for determining the reserves to be maintained
               by member banks of the Federal Reserve System for Eurocurrency
               Liabilities, as defined in Federal Reserve Board Regulation D,
               rounded upward to the nearest 1/100 of one percent.  The
               percentage will be expressed as a decimal, and will include, but
               not be limited to, marginal, emergency, supplemental, special,
               and other reserve percentages.

               (e) The Borrower may not elect an Eurodollar Rate with respect to
          any portion of the principal balance of any loan which is scheduled to
          be repaid before the last day of the applicable interest period.

               (f) Any portion of the principal balance of a loan already
          bearing interest at the Eurodollar Rate will not be converted to a
          different rate during its interest period.

               (g) Each prepayment of an Eurodollar Rate portion will be
          accompanied by the amount of accrued interest on the amount prepaid,
          and a prepayment fee equal to the amount (if any) by which

                    (i) the additional interest which would have been payable on
               the amount prepaid had it not been paid until the last day of the
               interest period, exceeds

                    (ii) the interest which would have been recoverable by the
               Bank by placing the amount prepaid on deposit in the London
               interbank market for a period starting on the date on which it
               was prepaid and ending on the last day of the interest period for
               such portion.

               (h) The Bank will have no obligation to accept an election for an
          Eurodollar Rate portion if any of the following described events has
          occurred and is continuing:

                                       3
<PAGE>

                    (i)  Dollar deposits in the principal amount, and for
               periods equal to the interest period, of an Eurodollar Rate
               portion are not available in the London interbank market; or

                    (ii) The Eurodollar Rate does not accurately reflect the
               cost of an Eurodollar Rate portion.

               (i) If at any time during any applicable interest period the
          Eurodollar Rate shall exceed the Maximum Rate and thereafter the
          Eurodollar Rate shall become less than the Maximum Rate, the rate of
          interest payable shall be the Maximum Rate until the Bank shall have
          received the amount of interest it otherwise would have received if
          the interest payable had not been limited by the Maximum Rate during
          the period of time the Eurodollar Rate exceeded the Maximum Rate.

               (j) Prior to receipt of the Compliance Certificate to be
          delivered with the Borrower's financial statements for the fiscal
          quarter ending September 30, 1999, the margin over the Libor Rate for
          the Line of Credit shall be One and a quarter percent (1.25%);
          thereafter, the "Applicable Libor Margin" shall be determined in
          accordance with the following table:

               ===========================================================
                   RATIO OF TOTAL LIABILITIES
                   TO TANGIBLE NET WORTH              LIBOR RATE MARGIN

                Less than 1.25 to 1.00                     0.875%
               -----------------------------------------------------------

                Greater than or equal to                    1.00%
                1.25 to 1.00 but less
                than 2.01 to 1.00
               -----------------------------------------------------------

                Greater than or equal to                   1.125%
                2.01 to 1.00 but less
                than 2.76 to 1.00
               -----------------------------------------------------------

                Greater than or equal to                    1.25%
                2.76 to 1.00
               ===========================================================

               Upon delivery of the Compliance Certificate pursuant to
          subsection 8.2(g) after the end of each fiscal quarter commencing with
          -----------------
          such Compliance Certificate delivered for the fiscal quarter ending
          November 30, 1999, the Applicable Libor Margin shall automatically be
          adjusted to the rate corresponding to the Ratio of Total Liabilities
          to Tangible Net Worth of Borrower set forth in the table above, such
          automatic adjustment to take effect prospectively the third Business
          Day after receipt by the Bank of the Compliance Certificate (the
          "Adjustment Date").  If Borrower fails to deliver such Compliance
          ----------------
          Certificate with respect to any fiscal quarter which

                                       4
<PAGE>

          sets forth such ratio within the period of time required by subsection
                                                                      ----------
          8.2(g), the Applicable Libor Margin shall automatically be adjusted to
          ------
          one and one quarter percent (1.25%) per annum. The automatic
          adjustments provided for in the preceding sentence shall take effect
          on the last day that the Compliance Certificate was required to be
          delivered and shall remain in effect until subsequently adjusted in
          accordance herewith upon the delivery of such Compliance Certificate.

     Section 2.4  Amendment to Section 4.2 of the Loan Agreement.  Effective as
                  ----------------------------------------------
of the date hereof, Section 4.2 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          4.2  Personal Property Supporting Guaranty.  The obligations of Aztec
               -------------------------------------
     Industries, Inc., Aztec Industries, Inc. - Moss Point, Automatic
     Processing, Incorporated, The Calvert Company, Inc., Gulf Coast
     Galvanizing, Inc., Arkgalv, Inc., Arbor-Crowley, Inc., Aztec Group Company,
     Aztec Holdings, Inc., Aztec Manufacturing Partnership, Ltd., Aztec
     Manufacturing-Waskom Partnership, Ltd., Rig-A-Lite Partnership, Ltd.,
     Atkinson Industries, Inc., Arizona Galvanizing, Inc., Hobson Galvanizing,
     Inc., International Galvanizers Partnership, Ltd., and Drilling Rig
     Electrical Systems Partnership, Ltd. (collectively, the "Guarantors") to
                                                              ----------
     the Bank will be secured by all equipment, fixtures, inventory, accounts,
     receivables and related assets the Guarantors now own or will own in the
     future, as further defined in the security agreement executed by the
     Guarantors; provided that the tem "Guarantors" shall include CGIT upon its
                 --------
     execution of a guaranty within  ninety (90) days of the acquisition of CGIT
     by the Borrower as provided by Section 1.4 of this Agreement.
                                    -----------

     Section 2.5  Amendment to Section 8.1 of the Loan Agreement.  Effective as
                  ----------------------------------------------
of the date hereof, Section 8.1 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          8.1  Use of Proceeds.  To use the proceeds of the Line of Credit only
               ---------------
     for the general working capital needs of the Borrower and its subsidiaries
     and to finance acquisitions permitted hereby, including,without limitation,
                                                   ---------
     the acquisition of CGIT.

     Section 2.6  Amendment to Section 8.4 of the Loan Agreement.  Effective as
                  ----------------------------------------------
of the date hereof, Section 8.4 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          8.4  Tangible Net Worth.  To maintain on a consolidated basis Tangible
               ------------------
     Net Worth equal to at least Eleven Million Dollars ($11,000,000, plus fifty
     percent (50%) of the Borrower's positive net income for each fiscal quarter
     of the Borrower ending after June 30, 1999 (no reductions to be made for
     any losses).  As used herein, the term "Tangible Net Worth" means the gross
                                             ------------------
     book value of the Borrower's assets (excluding goodwill, patents,
     trademarks, trade names, organization expense, treasury stock, unamortized
     debt discount and expense, deferred research and development costs,
     deferred marketing expenses, and other like intangibles, and monies due
     from affiliates, officers, directors or shareholders of the Borrower) less
     total liabilities, including but not limited to accrued and deferred income
     taxes, and any reserves against assets.

                                       5
<PAGE>

     Section 2.7  Amendment to Section 8.5 of the Loan Agreement. Effective as
                  ----------------------------------------------
of the date hereof, Section 8.5 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          8.5  Current and Long Term Liabilities to Tangible Net Worth.  To
               -------------------------------------------------------
     maintain on a consolidated basis a ratio of long term debt and Current
     Liabilities to Tangible Net Worth not exceeding 4.50 to 1.00 for the period
     from the date of the Sixth Amendment to this Agreement through February 28,
     2000, 3.00 to 1.00 thereafter through February 28, 2001, or 2.00 to 1.00
     thereafter.

     Section 2.8  Amendment to Section 8.6 of the Loan Agreement.  Effective as
                  ----------------------------------------------
of the date hereof, Section 8.6 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          8.6  Cash Flow Ratio.  To maintain on a consolidated basis a Cash Flow
               ---------------
     Ratio of at least 1.6:1.0.  As used herein, the term "Cash Flow Ratio"
                                                           ---------------
     means the ratio of Cash Flow to the current portion of long term debt plus
     interest expense.  "Cash Flow" is defined as net income from operations
                         ---------
     (before extraordinary or nonrecurring gains or losses), minus taxes, plus
     depreciation, amortization and other non-cash charges, plus interest
     expense, minus dividends.  This ratio will be calculated at the end of each
     fiscal quarter, using the results of that quarter and each of the 3
     immediately preceding quarters.  The current portion of long term debt will
     be measured as of the last day of the fiscal quarter preceding the date of
     calculation.

     Section 2.9  Amendment to Section 8.11 of the Loan Agreement. Effective as
                  -----------------------------------------------
of the date hereof, Section 8.1l of the Loan Agreement is hereby amended and
                    ------------
restated to read in its entirety as follows:

          8.11  Treasury Stock. Not to purchase, redeem, or otherwise acquire
                --------------
     for value any of its shares, or create any sinking fund in relation
     thereto.

     Section 2.10  Amendment to Section 8.22 of the Loan Agreement.  Effective
                   -----------------------------------------------
as of the date hereof, clause (c) of Section 8.22 of the Loan Agreement is
                                     ------------
hereby amended and restated to read in its entirety as follows:

                   (c) enter into any consolidation, merger, pool, joint
                   venture, syndicate, or other combination or purchase or
                   acquire all or substantially all the assets of another
                   business, except for such transaction if (i) the Borrower or
                   applicable Guarantor is the surviving entity; (ii) no default
                   exists hereunder or would result therefrom and before, and
                   after giving effect to, the proposed acquisition, the
                   representations and warranties set forth herein shall be true
                   and correct; and (iii) Borrower delivers to the Bank an
                   Environmental Questionnaire and Disclosure Statement in form
                   and disclosing such information as is acceptable to the Bank
                   relating to the assets or entity to be acquired prior to the
                   acquisition.

                                       6
<PAGE>

     Section 2.11  Amendment to Article 8 of the Loan Agreement.  Effective as
                   --------------------------------------------
of the date hereof, Article 8 of the Loan Agreement is hereby amended by adding
                    ---------
thereto new Sections 8.24 to read in its entirety as follows:
            -------------

          8.25  Funded Debt to EBITDA.  To maintain on a consolidated basis at
                ---------------------
     the end of each of the Borrower's fiscal quarters ending after the date of
     the Sixth Amendment to this Agreement a ratio of Funded Debt to EBITDA for
     the four consecutive fiscal quarters then ended not exceeding 3.00 to 1.00.
     As used herein, the term "Funded Debt" means all debt of the Borrower and
                               -----------
     its Subsidiaries for borrowed money (including capital lease obligations)
     and the term "EBITDA" means: (i) net income from operations (before
                   ------
     extraordinary or non-recurring gains or losses); plus (ii) any provision
                                                      ----
     for income or franchise taxes deducted in determining net income; plus
                                                                       ----
     (iii) interest expense deducted in determining net income; plus (iv)
                                                                ----
     depreciation and amortization expense deducted in determining net income;

     plus (v) other non cash charges deducted in determining net income.
     ----

                                   ARTICLE 3

                             Conditions Precedent
                             --------------------

     Section 3.1  Items to be Delivered By Borrower.  Prior to or simultaneously
                  ---------------------------------
with execution and delivery of this Amendment, the Borrower shall deliver, or
cause to be delivered, to the Bank the following:

          1   Officer's Certificates.  Due certification by the corporate
              ----------------------
     secretary (or other duly authorized officer acceptable to the Bank) of the
     Borrower and each other Pledging Party (or the general partner of any
     Pledging Party) (a) attaching a copy of resolutions duly adopted by its
     board of directors approving the terms and conditions contained in this
     Amendment, (b) certifying that the certificate of incorporation, bylaws,
     certificate of limited partnership, and other constituent documents of the
     Borrower, such other Pledging Party, or its general partner (as applicable)
     as previously certified to the Bank remain in full force and effect without
     amendment, and (c) including a certification of incumbency of all officers
     who are authorized to act in respect of the corporate resolutions
     referenced above, including the name, office, and signature of each such
     officer.

          2   Amendment Documents.  Each agreement, certificate, document, or
              -------------------
     instrument required by the Bank to be executed or delivered by the Borrower
     or any other Pledging Party in connection with this Amendment (the
     "Amendment Documents"), duly executed or delivered by the parties thereto.
      -------------------

     Section 3.2  Other Conditions.  The effectiveness of this Amendment is
                  ----------------
subject to the satisfaction of each of the additional following conditions
precedent:

          1   Continued Effect of Representations and Warranties.  All
              --------------------------------------------------
     representations and warranties contained in any loan document (including,
     without limitation, the Loan Agreement, as amended hereby; all of such loan
     documents are referred to collectively herein as the "Loan Documents")
                                                           --------------
     shall be true, correct, and complete in all material respects (as

                                       7
<PAGE>

     determined by the Bank in its sole discretion) except as disclosed
     otherwise to the Bank in writing and as acceptable to the Bank or
     representations specifically relating to a prior date or no longer relevant
     due to the occurrence of an event or circumstances specifically permitted
     hereunder or by any other Loan Document;

          2   Absence of Default.  No default or event of default shall have
              ------------------
     occurred and be continuing (after giving effect to this Amendment);

          3   Corporate Proceedings.  All corporate proceedings taken in
              ---------------------
     connection with the transactions contemplated by this Amendment and all
     other agreements, documents, and instruments executed and/or delivered
     pursuant hereto, and all legal matters incident thereto, shall be
     satisfactory to the Bank and its legal counsel; and

          4   Additional Information.  The Bank shall have received such
              ----------------------
     additional agreements, certificates, documents, instruments, and
     information as the Bank or its legal counsel, Jenkens & Gilchrist, a
     Professional Corporation, may reasonably request to effect the transactions
     contemplated hereby.

                                   ARTICLE 4

                        Representations and Warranties
                        ------------------------------

     Section 4.1  Representations and Warranties.  The Borrower hereby
                  ------------------------------
represents and warrants to the Bank that, as of the date of and after giving
effect to this Amendment:  (a) the execution, delivery, and performance of this
Amendment and any and all other Amendment Documents executed and/or delivered in
connection herewith have been authorized by all requisite corporate action on
the part of the Borrower and will not violate the Borrower's certificate of
incorporation or bylaws; (b) all representations and warranties set forth in the
Loan Agreement and in the Loan Documents are true and correct in all material
respects as if made again on and as of such date (except as disclosed otherwise
to the Bank in writing and as acceptable to the Bank or representations
specifically relating to a prior date or no longer relevant due to the
occurrence of an event or circumstances specifically permitted hereunder or by
any other Loan Document); (c) no default or event of default has occurred and is
continuing; and (d) the Loan Agreement and the other Loan Documents (as amended
by this Amendment) are and remain legal, valid, binding, and enforceable
obligations of the Borrower.

                                   ARTICLE 5

                                 Miscellaneous
                                 -------------

                                       8
<PAGE>

     Section 5.1  Governing Law.   THIS AMENDMENT, AND ALL DOCUMENTS AND
                  -------------
INSTRUMENTS EXECUTED IN CONNECTION HEREWITH, SHALL BE GOVERNED BY AND CONSTRUED
ACCORDING TO THE LAWS OF THE STATE OF TEXAS, PROVIDED THAT TO THE EXTENT FEDERAL
                                             --------
LAW WOULD ALLOW A HIGHER RATE OF INTEREST THAN WOULD BE ALLOWED BY THE LAWS OF
THE STATE OF TEXAS, THEN WITH RESPECT TO THE PROVISIONS OF ANY LAW WHICH
PURPORTS TO LIMIT THE AMOUNT OF INTEREST THAT MAY BE CONTRACTED FOR, CHARGED OR
RECEIVED IN CONNECTION WITH ANY OF THE OBLIGATIONS, SUCH FEDERAL LAW SHALL
APPLY.

     Section 5.2  Agreement Remains in Effect; No Waiver.  Except as expressly
                  --------------------------------------
provided herein, all terms and provisions of the Loan Agreement and the other
Loan Documents shall remain unchanged and in full force and effect and are
hereby ratified and confirmed.  No delay or omission by the Bank in exercising
any power, right, or remedy shall impair such power, right, or remedy or be
construed as a waiver thereof or an acquiescence therein, and no single or
partial exercise of any such power, right, or remedy shall preclude other or
further exercise thereof or the exercise of any other power, right, or remedy
under the Loan Agreement, the Loan Documents, or otherwise.

     Section 5.3  Survival of Representations and Warranties.  All
                  ------------------------------------------
representations and warranties made in this Amendment or any other Loan Document
shall survive the execution and delivery of this Amendment and the other Loan
Documents, and no investigation by the Bank or any closing shall affect the
representations and warranties or the right of the Bank to rely upon them.

     Section 5.4  Reference to Loan Agreement and Loan Documents.  Each of the
                  ----------------------------------------------
Loan Documents, including the Loan Agreement, the Amendment Documents, and any
and all other agreements, documents, or instruments now or hereafter executed
and/or delivered pursuant to the terms hereof or pursuant to the terms of the
Loan Agreement, as amended hereby, are hereby amended so that any reference in
the Loan Documents to the Loan Agreement shall mean a reference to the Loan
Agreement as amended hereby, and the term "loan documents" as used in the Loan
Agreement and as used in any of the other Loan Documents includes, without
limitation, the Amendment Documents.

     Section 5.5  Severability.  Any provision of this Amendment held by a court
                  ------------
of competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

     Section 5.6  Successors and Assigns.  This Amendment is binding upon and
                  ----------------------
shall inure to the benefit of the Borrower, the Bank, and their respective
successors in interest and assigns.  The Borrower may not assign any right,
power, duty, or obligation hereunder without the prior written consent of the
Bank.

     Section 5.7  Headings.  The headings, captions, and arrangements used in
                  --------
this Amendment are for convenience only and shall not affect the interpretation
of this Amendment.

                                       9
<PAGE>

     Section 5.8  Expenses of the Bank.  As provided in the Loan Agreement, the
                  --------------------
Borrower agrees to pay on demand all reasonable, third party out-of-pocket costs
and expenses incurred by the Bank in connection with the preparation,
negotiation, and execution of this Amendment, the Amendment Documents, or the
other Loan Documents executed pursuant hereto and any and all amendments,
modifications, and supplements thereto, including, without limitation, the
reasonable fees of the Bank's legal counsel, and all reasonable costs and
expenses incurred by the Bank in connection with the enforcement or preservation
of any rights under the Loan Agreement, as amended hereby, or any other Loan
Document, including, without limitation, the reasonable costs and fees of the
Bank's legal counsel.

     Section 5.9  Counterparts.  This Amendment may be executed simultaneously
                  ------------
in one or more multiple originals, each of which shall be deemed an original,
but all of which together shall constitute one and the same agreement.

     THIS AMENDMENT, TOGETHER WITH THE LOAN AGREEMENT AND THE OTHER LOAN
     DOCUMENTS AS WRITTEN, REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND
     MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR
     SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL
     AGREEMENTS BETWEEN THE PARTIES.

     IN WITNESS WHEREOF, the Borrower and the Bank have caused this Amendment to
be executed and delivered by their duly authorized officers effective as of the
date first written above.

                                    BORROWER:
                                    --------

                                    AZTEC MANUFACTURING CO.

                                    By: /s/ DANA PERRY
                                        ---------------------------------------
                                    Name: DANA PERRY
                                          -------------------------------------

                                    Title: V.P. FINANCE
                                           ------------------------------------

                                    BANK:
                                    ----

                                    BANK OF AMERICA, N.A. (successor by
                                    merger to Bank of America, Texas, N.A.)

                                    By: /s/ VINCE LIBERIO
                                        ---------------------------------------
                                    Name: VINCE LIBERIO
                                          -------------------------------------
                                    Title: SENIOR V.P.
                                           ------------------------------------

                                       10<PAGE>

                                EXHIBIT 10(21)
                  Eight Amendment to Business Loan Agreement
  between Registrant and Bank of America, Texas, N.A., dated January 31, 2000
<PAGE>

                  EIGHTH AMENDMENT TO BUSINESS LOAN AGREEMENT
                          (RECEIVABLES AND INVENTORY)

     This EIGHTH AMENDMENT TO BUSINESS LOAN AGREEMENT (RECEIVABLES AND
INVENTORY) (this "Amendment") is executed and entered into on January 31, 2000,
                  ---------
and between Bank of America, N.A. (successor by merger to Bank of America,
Texas, N.A., the "Bank") and Aztec Manufacturing Co. (the "Borrower").
                  ----                                     --------

                                   RECITALS:

     A.   The Borrower and the Bank are parties to that certain Business Loan
Agreement (Receivables and Inventory) dated as of June 28, 1996 (as amended and
as the same may be further amended, renewed, extended, restated, or otherwise
modified from time to time, the "Loan Agreement") pursuant to which the Bank
                                 --------------
agreed to provide to the Borrower a secured revolving credit facility and a
secured term loan facility.

     B.   The indebtedness of the Borrower to the Bank pursuant to the Loan
Agreement is secured by liens on the Borrower's property as described in that
certain Security Agreement: Receivables, Inventory and Equipment (Borrower);

     C.   Each subsidiary of the Borrower is a Pledging Party and a Guarantor
under the terms of the Loan Agreement.

     D.   The Borrower has requested that the Bank provide additional loans to
the Borrower as set forth herein and amend certain provisions of the Loan
Agreement, and, subject to satisfaction of the conditions set forth herein, the
Bank is willing to make such additional loans available to the Borrower and
amend the Loan Agreement.

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
contained herein and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto hereby agree as
follows:

                                   ARTICLE 1

                                  Definitions
                                  -----------

     Section 1.1  Definitions.  Unless otherwise defined in this Amendment, each
                  -----------
capitalized term used in this Amendment has the meaning given to such term in
the Loan Agreement (as amended by this Amendment).
<PAGE>

                                   ARTICLE 2

                                  Amendments
                                  ----------

     Section 2.1  Amendment to subsection 1.1(a) of the Loan Agreement.
                  ----------------------------------------------------
Effective as of the date hereof, subsection 1.1(a) of the Loan Agreement is
                                 -----------------
hereby amended and restated to read in its entirety as follows:

             (a)  Twenty Million Dollars ($20,000,000.00), or

     Section 2.2  Amendment to subsection 1.1(b) of the Loan Agreement.
                  ----------------------------------------------------
Effective as of the date hereof, clause (iii) of subsection 1.1(b) of the Loan
                                                 -----------------
Agreement is hereby amended and restated to read in its entirety as follows:

           (iii)  Sixty percent (60%) of the value of Acceptable Inventory
consisting of raw zinc provided that the amount of the Borrowing Base available
under clause (ii) above and this clause (iii) shall never exceed the amount
available under clause (i) above.

     Section 2.3  Amendment to Definition of Acceptable Receivable.  Effective
                  ------------------------------------------------
as of the date hereof, clause (g)(iv) of the definition of "Acceptable
Receivable" in Section 1.2 of the Loan Agreement is hereby amended and restated
               -----------
to read in its entirety as follows:

            (iv)  any person or entity located in a foreign country unless the
                  account is supported by a letter of credit issued by a bank
                  acceptable to the Bank or an insurance policy issued by an
                  insurance company acceptable to the Bank provided that the
                  amount of accounts receivable supported by insurance policies
                  may not exceed One Million Dollars ($1,000,000) in the
                  aggregate at any one time.

     Section 2.4  Further Amendment to Definition of Acceptable Receivable.
                  --------------------------------------------------------
Effective as of the date hereof, the definition of "Acceptable Receivable" in
Section 1.2 of the Loan Agreement is hereby amended by adding thereto a new
-----------
clause (n) to read in its entirety as follows:

             (n)  The account is otherwise acceptable to the Bank.

     Section 2.5  Amendment to Section 1.4 of the Loan Agreement. Effective as
                  ----------------------------------------------
of the date hereof, Section 1.4 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

             1.4  "Pledging Party" means the Borrower and each subsidiary of the
                   --------------
     Borrower organized in the United States of America or any State or other
     subdivision thereof that is party to a security agreement in favor of the
     Bank pledging to the Bank such subsidiary's accounts receivable, inventory,
     equipment, and other related assets.

                                        2

<PAGE>

     Section 2.6  Amendments to Section 2 of the Loan Agreement. Effective as of
                  ---------------------------------------------
the date hereof, each of the following sections in Section 2 ("LOAN AMOUNTS
                                                   ---------
AND TERMS") of the Loan Agreement is hereby amended and restated to read in its
entirety as follows:

          2.2  Availability Period. The line of credit is available between the
               -------------------
     date of this Agreement and July 1, 2002 (the "Expiration Date") unless the
                                                   ---------------
     Borrower is in default.

                                      ***

          2.6  Term Loans. The Bank agrees to provide term loans to the Borrower
     (the "Term Commitment") in the amounts of (a) Seventeen Million Five
           ---------------
     Hundred Thousand Dollars ($17,500,000.00) (the "Term Loan A") and (b) Eight
                                                     -----------
     Million Six Hundred Ninety Thousand Four Hundred Seventy-Six Dollars and
     Eighteen Cents ($8,690,476.18) (the "Term Loan B"; the Term Loan A and the
     Term Loan B are referred to collectively herein as the "Term Loans").
     -----------                                             ----------

          2.7  Availability Period.  The Term Commitment is available in two
               -------------------
     disbursements as follows:

               (a)  the Term Loan A shall be made in one disbursement from the
                    Bank on the date of the Eighth Amendment to this Agreement,
                    and

               (b)  the Term Loan B shall be made in one disbursement from the
                    Bank on the date of the Eighth Amendment to this Agreement.

          2.8  Purpose.  The line of credit provided pursuant to Section 2.1
               -------
shall be used for working capital purposes. The Term Loan A shall be used to
refinance the Term Loan A existing under this Agreement prior to the execution
of the Eighth Amendment to this Agreement and outstanding on the effective date
of such Eighth Amendment, and to fund the acquisition of Westside Coating
Services, Inc. The Term Loan B shall be used to refinance the Term Loan B
existing under this Agreement prior to the execution of the Eighth Amendment to
this Agreement and outstanding on the effective date of such Eighth Amendment.

          2.9  Term Loan Repayment Terms; Prepayment.
               -------------------------------------

               (a) (i)  The Borrower will repay the principal amount of the Term
               Loan A in seventy-three (73) successive monthly installments of
               Two Hundred Thirty-Nine Thousand, Seven Hundred Twenty-Six
               Dollars and Three Cents ($239,726.03) each, on the first business
               day of each calendar month, starting February 1, 2000. On March
               1, 2006, the Borrower will repay the remaining principal balance
               of the Term Loan A.

                   (ii) The Borrower will repay the principal amount of the Term
               Loan B in seventy-three (73) successive monthly installments of
               One Hundred Nineteen Thousand Forty-Seven Dollars and Sixty-Two
               Cents ($119,047.62) each, on the first business day of each
               calendar month, starting February 1, 2000. On March 1, 2006, the
               Borrower will repay the remaining principal balance of the Term
               Loan B.

                                       3
<PAGE>

          (b)  The Borrower may prepay either of the Term Loans in full or in
          part at any time in an amount not less than One Hundred Thousand
          Dollars ($100,000.00) or an integral multiple thereof.  Any such
          prepayment will be applied to the Term Loan A or the Term Loan B, as
          designated by the Borrower, to the most remote installment of
          principal due under this Agreement.

                                      ***

     2.13 [Reserved]

     2.14 Libor Rate.  The Borrower may elect to have all or portions of the
          ----------
principal balance of the loans bear interest at a rate equal to the lesser of
(i) the Maximum Rate or (ii) the Libor Rate plus (i) the Applicable Libor Margin
                                            ----
(as determined as provided in clause (j) below) with respect to the Line of
Credit and Term Loan A or (ii) one and one-quarter percent (1.25%) with respect
to the Term Loan B (the "Eurodollar Rate"), subject to the following
                         ---------------
requirements:

          (a)  The interest period during which the Eurodollar Rate will be in
     effect will be (i) 1, 2, 3, or 6 months with respect to the Line of Credit
     or (ii) 1, 2, 3, 6, or 12 months with respect to the Term Loans. The last
     day of the interest period will be determined by the Bank using the
     practices of the London interbank market.

          (b)  Each Eurodollar Rate portion under a loan will be for an amount
     not less than Five Hundred Thousand Dollars ($500,000.00) or an integral
     multiple thereof.

          (c)  The Borrower shall irrevocably request a Eurodollar Rate portion
     no later than 11:00 a.m. Dallas, Texas time two (2) banking days before the
     commencement of the interest period.

          (d)  The "LIBOR Rate" means the interest rate determined by the
                    ----------
     following formula, rounded upward to the nearest 1/100 of one percent.
     (All amounts in the calculation will be determined by the Bank as of the
     first day of the interest period.)

                    LIBOR Rate =          London Rate
                                  -----------------------------
                                   (1.00 - Reserve Percentage)
     Where,

                    (i)  "London Rate" means the interest rate (rounded upward
                          -----------
          to the nearest 1/16th of one percent) at which the Bank's London
          Branch, London, Great Britain, would offer U.S. dollar deposits for
          the applicable interest period to other major banks in the London
          interbank market at approximately 11:00 a.m. London time two (2)
          banking days prior to the commencement of the interest period.

                    (ii) "Reserve Percentage" means the total of the maximum
                          ------------------
          reserve percentages for determining the reserves to be maintained by

                                       4
<PAGE>

          member banks of the Federal Reserve System for Eurocurrency
          Liabilities, as defined in Federal Reserve Board Regulation D, rounded
          upward to the nearest 1/100 of one percent. The percentage will be
          expressed as a decimal, and will include, but not be limited to,
          marginal, emergency, supplemental, special, and other reserve
          percentages.

          (e)  The Borrower may not elect an Eurodollar Rate with respect to any
     portion of the principal balance of any loan which is scheduled to be
     repaid before the last day of the applicable interest period.

          (f)  Any portion of the principal balance of a loan already bearing
     interest at the Eurodollar Rate will not be converted to a different rate
     during its interest period.

          (g)  Each prepayment of an Eurodollar Rate portion will be accompanied
     by the amount of accrued interest on the amount prepaid, and a prepayment
     fee equal to the amount (if any) by which

               (i)  the additional interest which would have been payable on the
          amount prepaid had it not been paid until the last day of the interest
          period, exceeds

               (ii)  the interest which would have been recoverable by the Bank
          by placing the amount prepaid on deposit in the London interbank
          market for a period starting on the date on which it was prepaid and
          ending on the last day of the interest period for such portion.

          (h)  The Bank will have no obligation to accept an election for an
     Eurodollar Rate portion if any of the following described events has
     occurred and is continuing:

               (i)   Dollar deposits in the principal amount, and for periods
          equal to the interest period, of an Eurodollar Rate portion are not
          available in the London interbank market; or

               (ii)  The Eurodollar Rate does not accurately reflect the cost of
          an Eurodollar Rate portion.

          (i)  If at any time during any applicable interest period the
     Eurodollar Rate shall exceed the Maximum Rate and thereafter the Eurodollar
     Rate shall become less than the Maximum Rate, the rate of interest payable
     shall be the Maximum Rate until the Bank shall have received the amount of
     interest it otherwise would have received if the interest payable had not
     been limited by the Maximum Rate during the period of time the Eurodollar
     Rate exceeded the Maximum Rate.

          (j)  Prior to receipt of the Compliance Certificate to be delivered
     with the Borrower's financial statements for the fiscal quarter ending
     February 28, 2000, the margin over the Libor Rate for the Line of Credit
     and Term Loan A shall be one and one-quarter

                                       5
<PAGE>

        percent (1.25%); thereafter, the "Applicable Libor Margin" shall be
        determined quarterly in accordance with the following table:

               ================================================
                 RATIO OF FUNDED DEBT TO
                         EBITDA                     LIBOR RATE
                                                      MARGIN
               ------------------------------------------------
                 Less than or equal to 1.25            1.00%
                 to 1.00

               ------------------------------------------------
                 Greater than 1.25 to 1.00            1.125%
                 but less than or equal to
                 2.00 to 1.00
               ------------------------------------------------

                 Greater than 2.00 to 1.00            1.250%
                 but less than or equal to
                 2.75 to 1.00
               ------------------------------------------------
                 Greater than 2.75 to 1.00            1.500%
               ================================================

               Upon delivery of the Compliance Certificate pursuant to
        subsection 8.2(g) after the end of each fiscal quarter commencing with
        -----------------
        such Compliance Certificate delivered for the fiscal quarter ending
        February 28, 2000, the Applicable Libor Margin shall automatically be
        adjusted to the rate corresponding to the Ratio of Funded Debt to EBITDA
        of Borrower set forth in the table above, such automatic adjustment to
        take effect prospectively the third Business Day after receipt by the
        Bank of the Compliance Certificate (the "Adjustment Date"). If Borrower
                                                 ---------------
        fails to deliver such Compliance Certificate with respect to any fiscal
        quarter which sets forth such ratio within the period of time required
        by subsection 8.2(g), the Applicable Libor Margin shall automatically be
           -----------------
        adjusted to one and one half percent (1.50%) per annum. The automatic
        adjustments provided for in the preceding sentence shall take effect on
        the last day that the Compliance Certificate was required to be
        delivered and shall remain in effect until subsequently adjusted in
        accordance herewith upon the delivery of such Compliance Certificate.

     Section 2.7  Amendment to Section 3.1 of the Loan Agreement. Effective as
                  ----------------------------------------------
of the date hereof, Section 3.1 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

             3.1  Unused commitment fee. Subject to the provisions of Section
                  ---------------------
     11.12 hereof, the Borrower agrees to pay a fee on any difference between
     Twenty Million Dollars ($20,000,000) and the amount of the Line of Credit
     it actually uses, determined by the average loan balance maintained during
     the specified period. The fee will be calculated at one-quarter of one
     percent (0.25%) per year so long as the ratio of the Borrower's Funded Debt
     to EBITDA (determined as provided in Section 8.24) is less than or equal to
     2.75 to 1.00 and three-eighths of one percent (0.375%) per year for any
     period that such ratio exceeds 2.75 to 1.00. This fee is due on April 1,
     2000 and on the first day of each following January, April, July and
     October until the Expiration Date.

                                       6
<PAGE>

     Section 2.8  Amendment to Section 3.2 of the Loan Agreement. Effective as
                  ----------------------------------------------
of the date hereof, clause (c) of Section 3.2 of the Loan Agreement is hereby
                                  -----------
amended to read in its entirety as follows:

            (c)   The Borrower agrees to reimburse the Bank for the cost of
periodic audits of the collateral securing this Agreement; provided that, prior
to the occurrence of a default, such audits are not conducted at the expense of
the Borrower more than twice a year and the costs of each such audits do not
exceed $2,500 plus all out-of-pocket expenses of the Bank. The audits may be
performed by employees of the Bank or by independent auditors and may be made
more often than twice a year at the discretion of the Bank as provided in
Section 8.15 of this Agreement.

     Section 2.9   Amendment to Borrowing Base Certificate. Effective as of the
                   ---------------------------------------
date hereof, all references in the Borrowing Base Certificate (Exhibit B to the
                                                               ---------
Loan Agreement) to the maximum amount of the Line of Credit shall be changed to
Twenty Million Dollars ($20,000,000.00) and the amount of the Borrowing Base
attributable to Acceptable Inventory shall never exceed the amount thereof
attributable to Acceptable Receivables.

     Section 2.10  Amendment to Section 8.2 of the Loan Agreement. Effective as
                   ----------------------------------------------
of the date hereof, clause (b) of Section 8.2 of the Loan Agreement is hereby
amended to read in its entirety as follows:

             (b)   Within thirty days of the period's end, the Borrower's and
its subsidiaries' monthly financial statements. These financial statements may
be prepared by the Borrower. The statements shall be prepared on a consolidated
basis.

     Section 2.11  Amendment to Section 8.3 of the Loan Agreement. Effective as
                   ----------------------------------------------
of the date hereof, Section 8.3 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

             8.3   Hedging Term Loan A. The Borrower will, not later than July
                   -------------------
                   31, 2000, enter into a swap contract with (a) the Bank or an
                   affiliate thereof or (b) one or more other counterparties
                   rated in one of the two highest rating categories of Standard
                   & Poors Corporation or Moody's Investors Service, Inc., in
                   each case on market terms, to fix the interest rates with
                   respect to one hundred percent (100%) of the Term Loan A
                   outstanding under this Agreement as of such date. Such swap
                   contract shall be on terms acceptable to the Bank.

     Section 2.12  Amendment to Section 8.4 of the Loan Agreement. Effective as
                   ----------------------------------------------
of the date hereof, Section 8.4 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

             8.4   Tangible Net Worth.  To maintain on a consolidated basis
                   ------------------
     Tangible Net Worth equal to at least Ten Million Dollars ($10,000,000.00),
     plus fifty percent (50%) of the Borrower's positive net income for each
     fiscal quarter of the Borrower ending after January 31, 2000 (no reductions
     to be made for any losses). As used herein, the term "Tangible Net Worth"
                                                           ------------------
     means the gross book value of the Borrower's assets (excluding

                                       7
<PAGE>

     goodwill, patents, trademarks, trade names, organization expense, treasury
     stock, unamortized debt discount and expense, deferred research and
     development costs, deferred marketing expenses, and other like intangibles,
     and monies due from affiliates, officers, directors or shareholders of the
     Borrower) less total liabilities, including but not limited to accrued and
     deferred income taxes, and any reserves against assets.

     Section 2.13  Amendment to Section 8.5 of the Loan Agreement. Effective as
                   ----------------------------------------------
of the date hereof, Section 8.5 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

             8.5  Payment of Taxes and Claims. The Borrower will, and will cause
                  ---------------------------
                  each of its subsidiaries to, pay or discharge when due

                  (a)   all taxes, assessments, and governmental charges or
                        levies imposed upon it or upon its income or profits or
                        upon any properties belonging to it, except that real
                        property ad valorem taxes shall be deemed to have been
                        so paid or discharged if the same are paid before they
                        become delinquent, and

                  (b)   all lawful claims of materialmen, mechanics, carriers,
                        warehousemen, and landlords for labor, materials,
                        supplies, and rentals which, if unpaid, might become a
                        lien on any properties of the Borrower or any of its
                        subsidiaries;

                  except that this Section 8.5 shall not require the payment or
                  ------
                  discharge of any such tax, assessment, charge, levy, or claim
                  which is being contested in good faith by appropriate
                  proceedings and for which reserves in respect of the
                  reasonably anticipated liability therefor have been
                  appropriately established.

     Section 2.14 Amendment to Section 8.6 of the Loan Agreement. Effective as
                  ----------------------------------------------
of the date hereof, the first sentence of Section 8.6 of the Loan Agreement is
                                          -----------
hereby amended and restated to read in its entirety as follows:

          To maintain on a consolidated basis a Cash Flow Ratio of at least 1.30
     to 1.0 through the period ending February 28, 2000, or 1.25 to 1.00
     thereafter.

     Section 2.15 Amendment to Section 8.9 of the Loan Agreement. Effective as
                  ----------------------------------------------
of the date hereof, Section 8.9 of the Loan Agreement is hereby amended and
                    -----------
restated to read in its entirety as follows:

          8.9     Capital Expenditures. Not, and not permit any Guarantor,
                  --------------------
     without the Bank's prior written consent, to spend or incur obligations
     (including the total amount of any capital leases) of more than Six Million
     Five Hundred Thousand Dollars ($6,500,000.00) for the fiscal year ending
     February 28, 2000, Ten Million Dollars ($10,000,000.00) for the fiscal year
     ending February 28, 2001, and Four Million Five Hundred Thousand Dollars
     ($4,500,000.00) for each fiscal year ending February 28 thereafter in the
     aggregate for Borrower and all Guarantors in any single fiscal year to

                                       8
<PAGE>

     acquire fixed or capital assets, excluding from such aggregate amount any
     such expenditures financed pursuant to Section 8.7(d).

     Section 2.16  Amendment to Sections 8.24 and 8.25 of the Loan Agreement.
                   ---------------------------------------------------------
Effective as of the date hereof, Section 8.24 of the Loan Agreement is hereby
                                 ------------
deleted and Section 8.25 is redesignated as new Section 8.24 and is hereby
            ------------                        ------------
amended and restated to read in its entirety as follows:

          8.24  Funded Debt to EBITDA. To maintain on a consolidated basis at
                ---------------------
     the end of each of the Borrower's fiscal quarters ending after the date of
     the Eighth Amendment to this Agreement a ratio of Funded Debt to EBITDA for
     the four consecutive fiscal quarters then ended not exceeding 3.00 to 1.00
     through the period ending November 30, 2000 or 2.75 to 1.00 thereafter. As
     used herein, the term "Funded Debt" means all debt of the Borrower and its
                            -----------
     Subsidiaries for borrowed money (including capital lease obligations) and
     the term "EBITDA" means: (i) net income from operations (before
               ------
     extraordinary or non-recurring gains or losses); plus (ii) any provision
                                                      ----
     for income or franchise taxes deducted in determining net income; plus
                                                                       ----
     (iii) interest expense deducted in determining net income; plus (iv)
                                                                ----
     depreciation and amortization expense deducted in determining net income;
     plus (v) other non cash charges deducted in determining net income.
     ----

     Section 2.17  Amendment to Exhibit C to the Loan Agreement. Effective as of
                   --------------------------------------------
the date hereof, Exhibit C to the Loan Agreement is hereby amended and restated
                 ---------
to read in its entirety as set forth in Exhibit 1 to this Amendment.

                                   ARTICLE 3

                             Conditions Precedent
                             --------------------

     Section 3.1  Items to be Delivered By Borrower. Prior to or simultaneously
                  ---------------------------------
with execution and delivery of this Amendment, the Borrower shall deliver, or
cause to be delivered, to the Bank the following:

             1.   Officer's Certificates. Due certification by the corporate
                  ----------------------
     secretary (or other duly authorized officer acceptable to the Bank) of the
     Borrower and each other Pledging Party (or the general partner of any
     Pledging Party) (a) attaching a copy of resolutions duly adopted by its
     board of directors approving the terms and conditions contained in this
     Amendment, (b) certifying that the certificate of incorporation, bylaws,
     certificate of limited partnership, and other constituent documents of the
     Borrower, such other Pledging Party, or its general partner (as applicable)
     as previously certified to the Bank remain in full force and effect without
     amendment, and (c) including a certification of incumbency of all officers
     who are authorized to act in respect of the corporate resolutions
     referenced above, including the name, office, and signature of each such
     officer.

             2.   Amendment Documents. Each agreement, certificate, document, or
                  -------------------
     instrument required by the Bank to be executed or delivered by the Borrower
     or any other Pledging Party in connection with this Amendment (the
     "Amendment Documents"), duly executed or delivered by the parties thereto.
      -------------------

                                       9
<PAGE>

             3.   Reaffirmation. A Reaffirmation of Agreements executed by the
                  -------------
      Guarantors in the form attached hereto.

             4.   Payment of Fees and Expenses. Payment of all fees and expenses
                  ----------------------------
     of or incurred by the Bank and its counsel to the extent billed on or
     before the date hereof and payable pursuant to this Amendment including,
     without limitation, an amendment fee payable to the Bank in the amount of
     Twenty-Five Thousand Dollars ($25,000.00)

     Section 3.2  Other Conditions. The effectiveness of this Amendment is
                  ----------------
subject to the satisfaction of each of the additional following conditions
precedent:

             1.   Continued Effect of Representations and Warranties. All
                  --------------------------------------------------
     representations and warranties contained in any loan document (including,
     without limitation, the Loan Agreement, as amended hereby; all of such loan
     documents are referred to collectively herein as the "Loan Documents")
                                                           --------------
     shall be true, correct, and complete in all material respects (as
     determined by the Bank in its sole discretion) except as disclosed
     otherwise to the Bank in writing and as acceptable to the Bank or
     representations specifically relating to a prior date or no longer relevant
     due to the occurrence of an event or circumstances specifically permitted
     hereunder or by any other Loan Document;

             2.   Absence of Default. No default or event of default shall have
                  ------------------
     occurred and be continuing (after giving effect to this Amendment);

             3.   Year 2000 Compliance. The Bank shall be satisfied that (a) the
                  --------------------
     Borrower and its subsidiaries have taken all necessary and appropriate
     steps to ascertain the extent of, and to quantify and successfully address,
     business and financial risks facing the Borrower and its subsidiaries as a
     result of failure to become Year 2000 compliant (that is, that computer
     applications, imbedded microchips and other systems will be able to perform
     date-sensitive functions prior to and after December 31, 1999) including
     risks resulting from the failure of key vendors and suppliers of the
     Borrower and its subsidiaries to become Year 2000 compliant, and (b) the
     Borrower's and its subsidiaries' material computer applications and those
     of its key vendors and suppliers have, on a timely basis, adequately
     addressed the Year 2000 problem in all material respects.

             4.   Corporate Proceedings. All corporate proceedings taken in
                  ---------------------
     connection with the transactions contemplated by this Amendment and all
     other agreements, documents, and instruments executed and/or delivered
     pursuant hereto, and all legal matters incident thereto, shall be
     satisfactory to the Bank and its legal counsel; and

             5.   Additional Information. The Bank shall have received such
                  ----------------------
     additional agreements, certificates, documents, instruments, and
     information as the Bank or its legal counsel, may reasonably request to
     effect the transactions contemplated hereby.

                                       10
<PAGE>

                                   ARTICLE 4

                        Representations and Warranties
                        ------------------------------

     Section 4.1  Representations and Warranties. The Borrower hereby represents
                  ------------------------------
and warrants to the Bank that, as of the date of and after giving effect to this
Amendment: (a) the execution, delivery, and performance of this Amendment and
any and all other Amendment Documents executed and/or delivered in connection
herewith have been authorized by all requisite corporate action on the part of
the Borrower and will not violate the Borrower's certificate of incorporation or
bylaws; (b) all representations and warranties set forth in the Loan Agreement
and in the Loan Documents are true and correct in all material respects as if
made again on and as of such date (except as disclosed otherwise to the Bank in
writing and as acceptable to the Bank or representations specifically relating
to a prior date or no longer relevant due to the occurrence of an event or
circumstances specifically permitted hereunder or by any other Loan Document);
(c) no default or event of default has occurred and is continuing; and (d) the
Loan Agreement and the other Loan Documents (as amended by this Amendment) are
and remain legal, valid, binding, and enforceable obligations of the Borrower.

                                   ARTICLE 5

                                 Miscellaneous
                                 -------------

     Section 5.1  Governing Law. THIS AMENDMENT, AND ALL DOCUMENTS AND
                  -------------
INSTRUMENTS EXECUTED IN CONNECTION HEREWITH, SHALL BE GOVERNED BY AND CONSTRUED
ACCORDING TO THE LAWS OF THE STATE OF TEXAS, PROVIDED THAT TO THE EXTENT FEDERAL
                                             --------
LAW WOULD ALLOW A HIGHER RATE OF INTEREST THAN WOULD BE ALLOWED BY THE LAWS OF
THE STATE OF TEXAS, THEN WITH RESPECT TO THE PROVISIONS OF ANY LAW WHICH
PURPORTS TO LIMIT THE AMOUNT OF INTEREST THAT MAY BE CONTRACTED FOR, CHARGED OR
RECEIVED IN CONNECTION WITH ANY OF THE OBLIGATIONS, SUCH FEDERAL LAW SHALL
APPLY.

     Section 5.2  Agreement Remains in Effect; No Waiver. Except as expressly
                  --------------------------------------
provided herein, all terms and provisions of the Loan Agreement and the other
Loan Documents shall remain unchanged and in full force and effect and are
hereby ratified and confirmed. No delay or omission by the Bank in exercising
any power, right, or remedy shall impair such power, right, or remedy or be
construed as a waiver thereof or an acquiescence therein, and no single or
partial exercise of any such power, right, or remedy shall preclude other or
further exercise thereof or the exercise of any other power, right, or remedy
under the Loan Agreement, the Loan Documents, or otherwise.

     Section 5.3  Survival of Representations and Warranties. All
                  ------------------------------------------
representations and warranties made in this Amendment or any other Loan Document
shall survive the execution and delivery of this Amendment and the other Loan
Documents, and no investigation by the Bank or any closing shall affect the
representations and warranties or the right of the Bank to rely upon them.

                                       11
<PAGE>

     Section 5.4  Reference to Loan Agreement and Loan Documents. Each of the
                  ----------------------------------------------
Loan Documents, including the Loan Agreement, the Amendment Documents, and any
and all other agreements, documents, or instruments now or hereafter executed
and/or delivered pursuant to the terms hereof or pursuant to the terms of the
Loan Agreement, as amended hereby, are hereby amended so that any reference in
the Loan Documents to the Loan Agreement shall mean a reference to the Loan
Agreement as amended hereby, and the term "loan documents" as used in the Loan
Agreement and as used in any of the other Loan Documents includes, without
limitation, the Amendment Documents.

     Section 5.5  Severability. Any provision of this Amendment held by a court
                  ------------
of competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

     Section 5.6  Successors and Assigns. This Amendment is binding upon and
                  ----------------------
shall inure to the benefit of the Borrower, the Bank, and their respective
successors in interest and assigns. The Borrower may not assign any right,
power, duty, or obligation hereunder without the prior written consent of the
Bank.

     Section 5.7  Headings. The headings, captions, and arrangements used in
                  --------
this Amendment are for convenience only and shall not affect the interpretation
of this Amendment.

     Section 5.8  Expenses of the Bank. As provided in the Loan Agreement, the
                  --------------------
Borrower agrees to pay on demand all reasonable, third party out-of-pocket costs
and expenses incurred by the Bank in connection with the preparation,
negotiation, and execution of this Amendment, the Amendment Documents, or the
other Loan Documents executed pursuant hereto and any and all amendments,
modifications, and supplements thereto, and all reasonable costs and expenses
incurred by the Bank in connection with the enforcement or preservation of any
rights under the Loan Agreement, as amended hereby, or any other Loan Document,
including, without limitation, the reasonable costs and fees of the Bank's legal
counsel (whether inside or outside counsel), the cost of any environmental
investigation and audit, appraisal, title insurance premiums and survey and
inspection fees.

     Section 5.9  Arbitration. Any controversy or claim between or among the
                  -----------
parties hereto including but not limited to those arising out of or relating to
this Amendment, the Loan Agreement, any of the Loan Documents or any related
instruments, agreements or documents including any claim based on or arising
from an alleged tort, shall be determined by binding arbitration in accordance
with the Federal Arbitration Act (or if not applicable, the applicable state
law), and the rules of practice and procedure for the arbitration of commercial
disputes of JAMS/Endispute, or any successor thereto, as supplemented by any
special rules set forth in any of the Loan Documents. Judgment upon any
arbitration award may be entered in any court having jurisdiction. Any party to
the Loan Agreement may bring an action, including a summary or expedited
proceeding, to compel arbitration of any controversy or claim to which the Loan
Agreement applies in any court having jurisdiction over such action.

                                       12
<PAGE>

     Section 5.10 Counterparts. This Amendment may be executed simultaneously in
                  ------------
one or more multiple originals, each of which shall be deemed an original, but
all of which together shall constitute one and the same agreement.

     THIS AMENDMENT, TOGETHER WITH THE LOAN AGREEMENT AND THE OTHER LOAN
     DOCUMENTS AS WRITTEN, REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND
     MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR
     SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL
     AGREEMENTS BETWEEN THE PARTIES.

     IN WITNESS WHEREOF, the Borrower and the Bank have caused this Amendment to
be executed and delivered by their duly authorized officers effective as of the
date first written above.

                                     BORROWER:
                                     --------

                                     AZTEC MANUFACTURING CO.

                                     By: /s/ DANA PERRY
                                         --------------------------------------
                                     Name: DANA PERRY
                                           ------------------------------------
                                     Title: V.P. FINANCE
                                            -----------------------------------

                                     BANK:
                                     ----

                                     BANK OF AMERICA, N.A. (successor by
                                     merger to Bank of America, Texas, N.A.)

                                     By: /s/ DANIEL BROWN
                                         --------------------------------------
                                     Name: DANIEL BROWN
                                           ------------------------------------
                                     Title: VICE PRESIDENT
                                            -----------------------------------

                                       13

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