<PAGE>
                                                                 EXHIBIT 4.18(c)


                  THIRD AMENDMENT TO REVOLVING CREDIT AGREEMENT
                               AND LIMITED WAIVER

         THIS THIRD AMENDMENT TO REVOLVING CREDIT AGREEMENT AND LIMITED WAIVER
(this "Amendment"), dated as of September 7, 2001, is among STERLING CHEMICALS,
INC., a Delaware corporation (the "Company"), STERLING CANADA, INC., a Delaware
corporation ("Canada"), STERLING PULP CHEMICALS US, INC., a Delaware corporation
("Pulp US"), STERLING PULP CHEMICALS, INC., a Georgia corporation ("Pulp"),
STERLING FIBERS, INC., a Delaware corporation ("Fibers"), STERLING CHEMICALS
ENERGY, INC., a Delaware corporation ("Energy"), and STERLING CHEMICALS
INTERNATIONAL, INC., a Delaware corporation (together with the Company, Canada,
Pulp US, Pulp, Fibers and Energy, collectively, the "Borrowers"), the several
Lenders (as such term is defined in the hereinafter described Credit Agreement)
parties to this Amendment, and THE CIT GROUP/BUSINESS CREDIT, INC., as
Administrative Agent for the Lenders (in such capacity, the "Administrative
Agent").

                                    RECITALS:

         A. The Borrowers are parties to a Revolving Credit Agreement dated as
of July 19, 2001 (as amended by the First Amendment to Revolving Credit
Agreement and Revolver Intercreditor Agreement dated as of August 17, 2001, the
Second Amendment to Credit Agreement and Limited Waiver dated as of August 29,
2001 (the "Second Amendment"), and as the same may be further amended, modified,
restated, supplemented, renewed, extended, increased, rearranged and/or
substituted from time to time, the "Credit Agreement") among the Borrowers, the
Administrative Agent, and the several Lenders from time to time parties thereto.
Capitalized terms used and not otherwise defined herein shall have the meanings
ascribed to them in the Credit Agreement.

         B. Prior to the Second Amendment, the Borrowers advised the
Administrative Agent that (i) events of default had occurred (x) with respect to
Section 10(1)(p) of the Canadian Facility, as a result of the Bankruptcy Court
not having entered the Final Order prior to the thirtieth (30th) day after the
date of the entry of the Interim Order, and (y) with respect to Section 10(1)(q)
of the Canadian Facility, as a result of the Bankruptcy Court not having entered
an order approving the assumption of the "Inter-Sterling" Material Agreements
listed in Schedule 9 thereof within 45 days of the Filing Date (such events of
default are hereinafter referred to collectively as the "Canadian Facility
Defaults"), and (ii) as a result of the Canadian Facility Defaults, Events of
Default had occurred pursuant to Section 8.1.5 of the Credit Agreement (such
Events of Default are hereinafter referred to as the "Cross Defaults").

         C. The Canadian Facility Defaults were waived by the lenders under the
Canadian Facility (the "Canadian Facility Lenders") until September 7, 2001, and
the Cross Defaults were waived by the Banks pursuant to the Second Amendment for
so long as the Canadian Facility Defaults were waived by the Canadian Facility
Lenders.




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         D. Pursuant to the Credit Agreement, the Maturity Date will occur on
September 7, 2001, unless by such date (i) the Bankruptcy Court has entered the
Final Order, or (ii) the Maturity Date has occurred or has been extended
pursuant to Section 10.1(e)(i) of the Credit Agreement.

         E. The Canadian Facility Lenders have extended their waiver of the
Canadian Facility Defaults until September 14, 2001.

         F. In order to allow further time to pursue the entry by the Bankruptcy
Court of the Final Order, the Borrowers have requested that the Lenders (i)
re-affirm the waivers of the Cross Defaults in connection with the extension by
the Canadian Facility Lenders of the Canadian Facility Defaults, and (ii) amend
the Credit Agreement to, among other things, extend the Maturity Date pursuant
to Section 10.1(e)(i) of the Credit Agreement.

         G. The Lenders parties to this Amendment (which Lenders constitute the
Lenders required to effect the amendments and re-affirm the waivers intended
hereby) are willing to so re-affirm the waivers of the Cross Defaults and to so
amend the Credit Agreement, subject to the performance and observance in full of
each of the covenants, terms and conditions, and in reliance upon all of the
representations and warranties of the Borrowers, set forth herein.

         NOW, THEREFORE, in consideration of the premises and the covenants,
terms, conditions, representations and warranties herein contained, the parties
hereto agree hereby as follows:

         SECTION 1. AMENDMENT TO MATURITY DATE DEFINITION. Subject to the
covenants, terms and conditions set forth herein and in reliance upon the
representations and warranties herein contained, the Borrowers and the several
Lenders parties to this Amendment (which Lenders constitute the Lenders required
to effect the following amendments) hereby agree to amend the Credit Agreement
as set forth below in this Section 1, effective as of the Amendment Effective
Date (as hereinafter defined).

         (a) AMENDMENT TO DEFINITION OF MATURITY DATE. The definition of
"Maturity Date" is deleted from Section 1.01 and the following definition is
inserted in lieu thereof:

                  "Maturity Date" means the earliest to occur of (a) September
         14, 2001, if the Final Order has not been entered prior to such date
         (unless the Maturity Date has been extended pursuant to Section
         10.1(e)(i) below), (b) the date on which the Bankruptcy Court enters a
         final order denying approval of the transactions contemplated in this
         Agreement (other than the Priming Lien), and (c) the second anniversary
         of the Effective Date of this Agreement (unless the Maturity Date has
         been extended pursuant to Section 10.1(e)(i) below).

         (b) AMENDMENT TO SECTION 5.2.3. The date "August 30, 2001" is hereby
deleted from clause (a)(ii) of Section 5.2.3. and the date "September 14" is
inserted in lieu thereof.

         SECTION 2. LIMITED WAIVERS. Subject to the covenants, terms and
conditions set forth herein and in reliance upon the representations and
warranties herein contained, the Borrowers and the several Lenders parties to
this Amendment (which Lenders constitute the




                                       2
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Lenders required to effect the following waivers) hereby re-affirm the waiver
given in the Second Amendment of the following Events of Default effective as of
the Amendment Effective Date:

         (a) the Event of Default under Section 8.1.5 of the Credit Agreement
arising as a result of the Bankruptcy Court not having entered the Final Order
prior to the thirtieth (30th) day after the date of the entry of the Interim
Order as required by Section 10(1)(p) of the Canadian Facility; and

         (b) the Event of Default under Section 8.1.5 of the Credit Agreement
arising as a result of the Bankruptcy Court not having entered an order
approving the assumption of the "Inter-Sterling" Material Agreements listed in
Schedule 9 thereof within 45 days of the Filing Date as required by Section
10(1)(q) of the Canadian Facility;

provided in each case that the foregoing Events of Default are waived only if
and only for so long as the Canadian Facility Defaults are waived by the lenders
under the Canadian Facility pursuant to documentation in form and substance
acceptable to the Administrative Agent.

         SECTION 3. CONDITIONS PRECEDENT. The parties hereto agree that this
Amendment and the amendments to the Credit Agreement and re-affirmation of
waivers contained herein shall not be effective until the satisfaction of each
of the following conditions precedent:

         (a) EXECUTION AND DELIVERY OF THIS AMENDMENT. The Administrative Agent
shall have received a copy of this Amendment executed and delivered by each of
the applicable Obligors and by each of the Lenders.

         (b) REPRESENTATIONS AND WARRANTIES. Each of the representations and
warranties made in this Amendment shall be true and correct on and as of the
Amendment Effective Date as if made on and as of such date, both before and
after giving effect to this Amendment (except to the extent such representations
and warranties relate expressly to an earlier date, which representations and
warranties are true and correct as of such earlier date).

         (c) PAYMENT OF FEES AND EXPENSES. The Borrowers shall have paid and the
Administrative Agent shall have received all costs and expenses of the
Administrative Agent in connection with the preparation, reproduction,
execution, and delivery of this Amendment and any other documents prepared in
connection herewith, including, without limitation, the reasonable fees and
out-of-pocket expenses of counsel for the Administrative Agent.

         (d) APPROVAL BY BANKRUPTCY COURT. The Administrative Agent shall have
received a copy (or such other evidence satisfactory to Administrative Agent) of
an order of the Bankruptcy Court, which order (i) as entered, shall be
acceptable in form and substance to the Administrative Agent, (ii) shall be
entered on or before September 7, 2001, authorizing the Borrowers to execute,
deliver and to perform their respective obligations under this Amendment, (iii)
shall be in full force and effect, and (iv) shall not have been stayed,
reversed, modified or amended in any respect.



                                       3
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         SECTION 4. REPRESENTATIONS AND WARRANTIES. To induce the Administrative
Agent and the several Lenders parties hereto to enter into this Amendment and to
agree to the amendments and to re-affirm the waivers contained herein, each of
the Borrowers represents and warrants to the Administrative Agent and the
Lenders as follows:

         (a) AUTHORIZATION; NO CONTRAVENTION. The execution, delivery and
performance by the applicable Obligors of this Amendment have been duly
authorized by all necessary partnership, corporate or limited liability company
action, as applicable, and do not and will not (i) contravene the terms of any
Organic Documents of any Obligor, (ii) conflict with or result in any breach or
contravention of, or (except as contemplated by the Loan Documents or as
otherwise permitted by the Credit Agreement) the creation of any Lien under, any
document evidencing any contractual obligation to which any Obligor is a party
or, upon entry of the approval of the Bankruptcy Court described in Section 3(d)
above, any order, injunction, writ or decree of any Governmental Authority to
which any Obligor is a party or its property is subject, or (iii) violate any
applicable law binding on or affecting any Obligor.

         (b) GOVERNMENTAL AUTHORIZATION. Upon entry of the approval of the
Bankruptcy Court described in Section 3(d) above, no approval, consent,
exemption, authorization or other action by, or notice to, or filing with or
approvals required under state blue sky securities laws or by any Governmental
Authority is necessary or required in connection with the execution, delivery,
performance or enforcement of this Amendment.

         (c) NO DEFAULT. No Default or Event of Default exists under any of the
Loan Documents other than the Cross Defaults. No Obligor is in default under or
with respect to (i) its Organic Documents or (ii) any material contractual
obligation of such Person other than the Canadian Facility Defaults. The
execution, delivery and performance of this Amendment shall not result in any
default under any contractual obligation of any Obligor in any respect.

         (d) BINDING EFFECT. Upon entry of the approval of the Bankruptcy Court
described in Section 3(d) above, this Amendment and the Credit Agreement as
amended hereby constitute the legal, valid and binding obligations of the
Obligors that are parties thereto, enforceable against such Obligors in
accordance with their respective terms, except as enforceability may be limited
by applicable bankruptcy, insolvency, or similar laws affecting the enforcement
of creditors' rights generally or by equitable principles of general
applicability.

         (e) REPRESENTATIONS AND WARRANTIES. The representations and warranties
set forth in the Credit Agreement and the other Loan Documents are true and
correct in all material respects on and as of the Amendment Effective Date, both
before and after giving effect to the amendments contemplated in this Amendment,
as if such representations and warranties were being made on and as of the
Amendment Effective Date (except to the extent such representations and
warranties relate expressly to an earlier date, which representations and
warranties are true and correct as of such earlier date, and except to the
extent that such representations and warranties would not be true and correct
solely as a result of the Canadian Facility Defaults and the Cross Defaults).



                                       4
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         SECTION 5. MISCELLANEOUS

         (a) RATIFICATION AND CONFIRMATION OF LOAN DOCUMENTS. Except for the
specific amendments and waivers expressly set forth in and re-affirmed by this
Amendment, the terms, provisions, conditions and covenants of the Credit
Agreement and the other Loan Documents remain in full force and effect and are
hereby ratified and confirmed, and the execution, delivery and performance of
this Amendment shall not in any manner operate as a waiver of, consent to or
amendment of any other term, provision, condition or covenant of the Credit
Agreement or any other Loan Document. Without limiting the generality of the
foregoing, the waivers described in Section 2 of this Amendment shall be limited
precisely as set forth above, and nothing in this Amendment shall be deemed (i)
to constitute a waiver of compliance or consent to noncompliance by any of the
Obligors with respect to any other term, provision, covenant or condition of the
Credit Agreement or other Loan Documents; (ii) to prejudice any right or remedy
that the Administrative Agent or the Lenders may now have or may have in the
future under or in connection with the Credit Agreement or any other Loan
Document; or (iii) to constitute a waiver of compliance or consent to
noncompliance by any Obligor with respect to the terms, provisions, covenants
and conditions of the Credit Agreement made subject hereof, other than as
specifically set forth herein and for the time periods specifically set forth
herein.

         (b) FEES AND EXPENSES. The Borrowers jointly and severally agree to pay
on demand all reasonable costs and expenses of the Administrative Agent in
connection with the preparation, reproduction, execution, and delivery of this
Amendment and any other documents prepared in connection herewith, including,
without limitation, the reasonable fees and out-of-pocket expenses of counsel
for the Administrative Agent.

         (c) HEADINGS. Section and subsection headings in this Amendment are
included herein for convenience of reference only and shall not constitute a
part of this Amendment for any other purpose or be given any substantive effect.

         (d) APPLICABLE LAW. THIS AMENDMENT SHALL BE GOVERNED BY, AND SHALL BE
CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK,
WITHOUT REGARD TO CONFLICTS OF LAW PRINCIPLES.

         (e) COUNTERPARTS AND AMENDMENT EFFECTIVE DATE. This Amendment may be
executed in any number of counterparts and by different parties hereto in
separate counterparts, each of which when so executed and delivered shall be
deemed an original, but all such counterparts together shall constitute but one
and the same instrument; signature pages may be detached from multiple separate
counterparts and attached to a single counterpart so that all signature pages
are physically attached to the same document. This Amendment shall become
effective when (i) each of the conditions precedent set forth in Section 3 of
this Amendment have been satisfied and (ii) the Administrative Agent has
received counterparts of this Amendment executed by the Borrowers, each of the
Obligors that are parties hereto and each of the Lenders (the "Amendment
Effective Date").



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<PAGE>

         (f) AFFIRMATION OF OBLIGATIONS. Notwithstanding that such consent is
not required thereunder, the undersigned Obligors hereby consent to the
execution and delivery of this Amendment by the parties hereto and reaffirm
their respective obligations under each of the Loan Documents to which such
Obligors are parties.

         (g) CONFIRMATION OF LOAN DOCUMENTS AND LIENS. As a material inducement
to the Lenders to amend the Credit Agreement and waive the Cross Defaults, the
Obligors that are parties hereto hereby (i) acknowledge and confirm the
continuing existence, validity and effectiveness of the Loan Documents to which
they are parties, including, without limitation the Security Documents, and the
Liens granted under the Security Documents, (ii) agrees that the execution,
delivery and performance of this Amendment shall not in any way release,
diminish, impair, reduce or otherwise affect such Loan Documents and Liens and
(iii) acknowledges and agrees that the Liens granted under the Security
Documents secure payment of the Obligations under the Loan Documents in the same
priority as on the date such Liens were created and perfected, and the
performance and observance by the Borrowers and the other Obligors of the
covenants, agreements and conditions to be performed and observed by each under
the Credit Agreement, as amended hereby.

         (h) FINAL AGREEMENT. THIS AMENDMENT, TOGETHER WITH THE CREDIT AGREEMENT
AND OTHER LOAN DOCUMENTS, REPRESENTS 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.


      [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK; SIGNATURE PAGES FOLLOW]




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         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed and delivered by their proper and duly authorized officers
effective as of the Amendment Effective Date.

                                   OBLIGORS:


                                   STERLING CHEMICALS, INC.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   STERLING CANADA, INC.


                                   By:
                                      ----------------------------------------
                                         Title:

                                   STERLING PULP CHEMICALS US, INC.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   STERLING PULP CHEMICALS, INC.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   STERLING FIBERS, INC.


                                   By:
                                      ----------------------------------------
                                         Title:





<PAGE>


                                   STERLING CHEMICALS ENERGY, INC.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   STERLING CHEMICALS
                                   INTERNATIONAL, INC.


                                   By:
                                      ----------------------------------------
                                         Title:



                                   STERLING CHEMICALS HOLDINGS, INC.



                                   By:
                                      ----------------------------------------
                                         Title:




<PAGE>

                                   ADMINISTRATIVE AGENT:

                                   THE CIT GROUP/BUSINESS CREDIT, INC.,
                                   as Administrative Agent


                                   By:
                                      ----------------------------------------
                                         Title:


                                   LENDERS:


                                   THE CIT GROUP/BUSINESS CREDIT, INC.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   IBJ WHITEHALL BUSINESS CREDIT
                                   CORPORATION


                                   By:
                                      ----------------------------------------
                                         Title:


                                   FLEET CAPITAL CORPORATION


                                   By:
                                      ----------------------------------------
                                         Title:


                                   TEXTRON FINANCIAL CORP.


                                   By:
                                      ----------------------------------------
                                         Title:





<PAGE>


                                   TRANSAMERICA BUSINESS CAPITAL CORPORATION


                                   By:
                                      ----------------------------------------
                                         Title:


                                   GMAC BUSINESS CREDIT, LLC


                                   By:
                                      ----------------------------------------
                                         Title:


                                   THE PROVIDENT BANK


                                   By:
                                      ----------------------------------------
                                   Title:


                                   GPSF SECURITIES, INC.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   FOOTHILL INCOME TRUST II, L.P.


                                   By:
                                      ----------------------------------------
                                         Title:


                                   CONGRESS FINANCIAL CORPORATION


                                   By:
                                      ----------------------------------------
                                         Title:


<PAGE>




                                   COMERICA BANK


                                   By:
                                      ----------------------------------------
                                         Title: