<PAGE>   1
                     U.S. Securities and Exchange Commission
                             Washington, D.C. 20549

                                    FORM 10-Q

[X]  QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
     EXCHANGE ACT OF 1934

                         For the Quarterly Period Ended
                                FEBRUARY 28, 1999

[ ]  TRANSITION REPORT UNDER SECTION 13 OR 15(d) OF THE SECURITIES
     EXCHANGE ACT OF 1934

                    For the Transition Period from N/A to N/A

                           Commission File No. 1-7755


                                SUMMA INDUSTRIES
                (Name of registrant as specified in its charter)


           DELAWARE                                        95-1240978
(State or other jurisdiction of                         (I.R.S. employer
 incorporation or organization)                       identification number)


        21250 HAWTHORNE BOULEVARD, SUITE 500, TORRANCE, CALIFORNIA 90503
          (Address of principal executive offices, including zip code)


                  Registrant's Telephone Number: (310) 792-7024


Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Exchange Act during the preceding 12
months (or for such shorter period that the registrant was required to file such
report), and (2) has been subject to such filing requirements for the past 90
days. Yes X No ___


The number of shares of common stock outstanding as of February 28, 1999 was
4,265,683.



<PAGE>   2

                                SUMMA INDUSTRIES

                                      INDEX

<TABLE>
<CAPTION>
                                                                                                  Page
                                                                                                  ----
        <S>                                                                                        <C>
        PART I - FINANCIAL INFORMATION

        Item 1.  Financial Statements:

                 Condensed Consolidated Balance Sheets -
                 August 31, 1998 and February 28, 1999 (unaudited) .................................3

                 Condensed Consolidated Statements of Income (unaudited) -
                 three months and six months ended February 28, 1998 and 1999.......................4

                 Consolidated Statements of Cash Flows (unaudited) -
                 six months ended February 28, 1998 and 1999........................................5

                 Notes to Condensed Consolidated Financial Statements (unaudited).................. 6

        Item 2.  Management's Discussion and Analysis
                 of Financial Condition and Results of Operations...................................8

        PART II - OTHER INFORMATION................................................................12

        Item 1.  Legal Proceedings.................................................................12
        Item 4.  Submission of Matters to a Vote of Security Holders...............................12
        Item 5.  Other Information.................................................................13
        Item 6.  Exhibits and Reports on Form 8-K..................................................17

        Signature Page.............................................................................17

</TABLE>




                                       2


<PAGE>   3

                                SUMMA INDUSTRIES

                      CONDENSED CONSOLIDATED BALANCE SHEETS

<TABLE>
<CAPTION>
 -------------------------------------------------------------------------------------------------------
                                                                      August 31, 1998  February 28, 1999
 ASSETS                                                                                      (unaudited)
 -------------------------------------------------------------------------------------------------------
<S>                                                                   <C>              <C>
 Current assets:
     Cash and cash equivalents                                            $   293,000        $   324,000
     Accounts receivable                                                   12,975,000         13,745,000
     Inventories                                                            9,392,000          9,964,000
     Prepaid expenses and other                                             1,439,000          1,376,000
 -------------------------------------------------------------------------------------------------------
        Total current assets                                               24,099,000         25,409,000
 -------------------------------------------------------------------------------------------------------
 Property, plant and equipment                                             27,796,000         28,959,000
    Less accumulated depreciation                                           7,132,000          8,923,000
 -------------------------------------------------------------------------------------------------------
         Net property, plant and equipment                                 20,664,000         20,036,000
 -------------------------------------------------------------------------------------------------------
 Other assets                                                               1,006,000          1,016,000
 Goodwill and other intangibles, net                                       18,214,000         17,942,000
 -------------------------------------------------------------------------------------------------------
       Total assets                                                       $63,983,000        $64,403,000
 =======================================================================================================
 LIABILITIES AND STOCKHOLDERS' EQUITY
 -------------------------------------------------------------------------------------------------------
 Current liabilities:
   Accounts payable                                                       $ 5,299,000        $ 5,083,000
   Accrued liabilities                                                      5,279,000          4,765,000
   Current maturities of long-term debt                                     2,667,000          3,030,000
 -------------------------------------------------------------------------------------------------------
      Total current liabilities                                            13,245,000         12,878,000
 -------------------------------------------------------------------------------------------------------
 Long-term debt, net of current maturities                                 18,675,000         16,755,000
 Other long-term liabilities                                                3,945,000          3,857,000
 -------------------------------------------------------------------------------------------------------
      Total liabilities                                                    35,865,000         33,490,000
 -------------------------------------------------------------------------------------------------------
 Stockholders' equity:
   Common stock, par value $.001; 10,000,000 shares authorized;
         issued and outstanding: 4,257,307 at August 31, 1998 and
         4,265,683 at February 28, 1999                                    18,505,000         18,541,000
   Retained earnings                                                        9,613,000         12,372,000
 -------------------------------------------------------------------------------------------------------
      Total stockholders' equity                                           28,118,000         30,913,000
 -------------------------------------------------------------------------------------------------------
      Total liabilities and stockholders' equity                          $63,983,000        $64,403,000
 =======================================================================================================
</TABLE>


 See accompanying notes to consolidated financial statements.



                                       3

<PAGE>   4

                                SUMMA INDUSTRIES

                   CONDENSED CONSOLIDATED STATEMENTS OF INCOME
                                   (unaudited)

<TABLE>
<CAPTION>
                                          Three months ended February 28   Six months ended February 28
-------------------------------------------------------------------------------------------------------
                                                  1998            1999              1998           1999
-------------------------------------------------------------------------------------------------------
<S>                                        <C>              <C>              <C>            <C>        
Net sales                                  $20,410,000      $22,987,000      $36,844,000    $46,258,000
Cost of sales                               14,445,000       15,727,000       25,794,000     31,732,000
-------------------------------------------------------------------------------------------------------
Gross profit                                 5,965,000       7,260,000        11,050,000     14,526,000
Selling, general, administrative and
   other expenses                            4,033,000       4,692,000         7,432,000      9,264,000
-------------------------------------------------------------------------------------------------------
Operating income from continuing
   operations                                1,932,000       2,568,000         3,618,000      5,262,000
Interest expense                               452,000         368,000           653,000        748,000
-------------------------------------------------------------------------------------------------------
Income from continuing operations before
    provision for taxes                      1,480,000       2,200,000         2,965,000      4,514,000
Provision for income taxes                     600,000         847,000         1,219,000      1,755,000
-------------------------------------------------------------------------------------------------------
Income from continuing operations              880,000       1,353,000         1,746,000      2,759,000
Income from discontinued operations,
  net of the effect of income tax               79,000              --           233,000             --
-------------------------------------------------------------------------------------------------------
Net income                                  $  959,000     $ 1,353,000       $ 1,979,000    $ 2,759,000
=======================================================================================================
Earnings per common share
-------------------------------------------------------------------------------------------------------
Basic
       Continuing operations                      $.21            $.32              $.42           $.65
       Discontinued operations                    $.02            $ --              $.06           $ --
       Net income                                 $.23            $.32              $.48           $.65
=======================================================================================================
Diluted
       Continuing operations                      $.20            $.30              $.40           $.62
       Discontinued operations                    $.02            $ --              $.05           $ --
       Net income                                 $.22            $.30              $.45           $.62
=======================================================================================================
Weighted average common shares
  outstanding                                  
       Basic                                 4,192,000       4,264,000         4,149,000      4,259,000
       Diluted                               4,446,000       4,445,000         4,357,000      4,439,000
 ------------------------------------------------------------------------------------------------------
</TABLE>


See accompanying notes to consolidated financial statements.


                                       4

<PAGE>   5

                                SUMMA INDUSTRIES

                      CONSOLIDATED STATEMENTS OF CASH FLOWS
                                   (unaudited)

<TABLE>
<CAPTION>
                                                                           Six months ended February 28
-------------------------------------------------------------------------------------------------------
                                                                                 1998              1999
-------------------------------------------------------------------------------------------------------
<S>                                                                      <C>               <C>
Operating activities:
Net income                                                               $  1,979,000      $  2,759,000
-------------------------------------------------------------------------------------------------------
Adjustments to reconcile net income to net cash provided by 
   operating activities:
    Depreciation                                                            1,528,000         1,840,000
    Amortization                                                              211,000           272,000
    Loss (gain) on disposition of property, plant and equipment               122,000           (12,000)
    Net change in assets and liabilities, net of effects from
       purchase of Calnetics in fiscal 1998:                              
       Accounts receivable                                                   (868,000)         (770,000)
       Inventories                                                           (236,000)         (572,000)
       Prepaid expenses and other assets                                       88,000            53,000
       Accounts payable                                                       669,000          (216,000)
       Accrued liabilities                                                   (732,000)         (602,000)
-------------------------------------------------------------------------------------------------------
          Total adjustments                                                   782,000            (7,000)
-------------------------------------------------------------------------------------------------------
         Net cash provided by operating activities                          2,761,000         2,752,000
-------------------------------------------------------------------------------------------------------
Investing activities:
Acquisition of business (Note 5)                                          (20,326,000)               --
Purchases of property and equipment                                        (1,461,000)       (1,200,000)
Net decrease in unexpended revenue bond proceeds                              371,000                --
-------------------------------------------------------------------------------------------------------
          Net cash (used in) investing activities                         (21,416,000)       (1,200,000)
-------------------------------------------------------------------------------------------------------
Financing activities:
Net proceeds from line of credit                                            5,985,000         1,966,000
Proceeds from issuance of long-term debt                                   13,500,000           727,000
Payments on long-term debt                                                 (4,194,000)       (4,250,000)
Proceeds from the exercise of stock options                                   804,000           189,000
Purchases of common stock                                                          --          (153,000)
-------------------------------------------------------------------------------------------------------
          Net cash provided by (used in) financing  activities             16,095,000        (1,521,000)
-------------------------------------------------------------------------------------------------------
 Net increase (decrease) in cash and cash equivalents                      (2,560,000)           31,000
 Cash and cash equivalents, beginning of period                             2,883,000           293,000
-------------------------------------------------------------------------------------------------------
 Cash and cash equivalents, end of period                                 $   323,000       $   324,000
=======================================================================================================
</TABLE>

See accompanying notes to consolidated financial statements.



                                       5

<PAGE>   6


                                SUMMA INDUSTRIES

              NOTES TO CONDENSED CONSOLIDATED FINANCIAL STATEMENTS
                                   (unaudited)

1.      BASIS OF PRESENTATION

        The accompanying condensed consolidated financial statements of Summa
Industries (the "Company"), some of which are unaudited, have been condensed in
certain respects and should, therefor, be read in conjunction with the audited
financial statements and notes related thereto contained in the Company's Annual
Report on Form 10-K for the year ended August 31, 1998. In the opinion of the
Company, the accompanying unaudited interim consolidated financial statements
contain all adjustments necessary for a fair presentation for the interim
period, all of which were normal recurring adjustments. The results of
operations for the six months ended February 28, 1999 are not necessarily
indicative of the results to be expected for the full year ending August 31,
1999.

2.      INVENTORIES

        Inventories were as follows:
<TABLE>
<CAPTION>
                                           August 31, 1998          February 28, 1999
                                                                          (unaudited)
                    ------------------------------------------------------------------
                    <S>                    <C>                        <C>
                    Finished goods              $3,611,000                 $3,696,000
                    Work in process                111,000                    160,000
                    Materials and parts          5,670,000                  6,108,000
                    ------------------------------------------------------------------
                                                $9,392,000                 $9,964,000
                    ==================================================================
</TABLE>

3.    DILUTED EARNINGS PER SHARE

      Diluted earnings per share were calculated using the "treasury stock"
method as if dilutive stock options had been exercised and the funds were used
to purchase common shares at the average market price during the period.

<TABLE>
<CAPTION>

                                                                  Three months ended        Six months ended
                                                                         February 28             February 28
           -------------------------------------------------------------------------------------------------
                                                                    1998        1999         1998       1999
           -------------------------------------------------------------------------------------------------
<S>                                                            <C>         <C>          <C>        <C>      
           Weighted average shares outstanding  - basic        4,192,000   4,264,000    4,149,000  4,259,000
           Effect of dilutive securities
              Impact of common shares to be issued under
                 stock option plans                              254,000     181,000      208,000    180,000
           -------------------------------------------------------------------------------------------------
           Weighted average shares outstanding-diluted         4,446,000   4,445,000    4,357,000  4,439,000
           =================================================================================================
</TABLE>




                                       6

<PAGE>   7

4.  SUPPLEMENTAL CASH FLOW INFORMATION

<TABLE>
<CAPTION>
                                                                       Six months ended February 28
              -------------------------------------------------------------------------------------
                                                                            1998               1999
              -------------------------------------------------------------------------------------
              <S>                                                     <C>                <C>
              Cash paid during the period:
                 Interest                                           $    752,000         $  771,000
                 Income taxes                                       $  1,745,000         $2,380,000
              -------------------------------------------------------------------------------------

              -------------------------------------------------------------------------------------
              Non-cash investing and financing activities:
              -------------------------------------------------------------------------------------
              Details of acquisition
                 Fair value of assets acquired                      $ 31,792,000
                 Liabilities assumed or incurred                      (8,821,000)
                 Value of options issued                              (1,345,000)
              -------------------------------------------------------------------------------------
              Cash paid                                               21,626,000
              Less cash acquired                                       1,300,000
              =====================================================================================
              Net cash used in acquisition                          $ 20,326,000
              =====================================================================================
</TABLE>

        The liabilities assumed or incurred ($8,821,000) in the preceeding table
includes a $709,000 obligation to acquire Calnetics shares outstanding as of
February 28, 1998. At February 28, 1999, the remaining obligation to acquire
Calnetics shares was $223,000.

5.   ACQUISITION

        On October 28, 1997, the Company completed the acquisition of Calnetics
Corporation ("Calnetics"). The total acquisition cost was $31,792,000,
consisting of cash due to former Calnetics shareholders of $22,335,000,
acquisition costs of $50,000, liabilities assumed or incurred of $8,062,000 and
an estimated fair value of $1,345,000 for options issued in conjunction with the
transaction, primarily replacement options issued to Calnetics employees who
continued with the Company. The acquisition has been accounted for using the
purchase method of accounting and, accordingly, the purchase price has been
allocated to identifiable tangible and intangible assets purchased and
liabilities assumed or incurred based upon their fair value at the date of
acquisition. The excess of the purchase price over the fair value of the net
assets acquired amounted to $13,974,000 and has been recorded as goodwill which
is being amortized on a straight-line basis over 40 years.

        The results of operations of Calnetics have been included in the
consolidated results of operations and the consolidated statements of cash flows
of the Company since October 28, 1997, the date of the acquisition. The
following pro forma financial information presents the results of operations of
the continuing businesses of the Company with Calnetics as though the
acquisition of Calnetics had been made as of September 1, 1997. Pro forma
adjustments have been made to give the effect to the amortization of goodwill,
adjustments in depreciation and inventory value, a reduction in redundant
operating expense, interest expense related to acquisition debt, the related tax
effects and the effect upon basic and diluted earnings per share of the stock
options issued in conjunction with the acquisition.




                                       7


<PAGE>   8
\
<TABLE>
<CAPTION>

                                     Three months ended February 28    Six months ended February 28  
---------------------------------------------------------------------------------------------------  
                                               1998            1999             1998           1999  
---------------------------------------------------------------------------------------------------  
<S>                                     <C>             <C>              <C>            <C>          
Net sales                               $20,410,000     $22,987,000      $42,562,000    $46,258,000  
Income from continuing operations       $   880,000     $ 1,353,000      $ 1,854,000    $ 2,759,000  
===================================================================================================  
Income from continuing operations                                                                    
    per common share                                                                                 
        basic                                  $.21            $.32             $.45           $.65  
        diluted                                $.20            $.30             $.43           $.62  
===================================================================================================  
</TABLE>


        The pro forma results in the preceding table are not necessarily
indicative of what the actual consolidated results of operations might have been
if the acquisition had been effective at September 1, 1997 or the results which
may be achieved in the future.

MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS 
OF OPERATIONS

        Statements contained in this Quarterly Report on Form 10-Q, which are
not purely historical, are forward-looking statements within the meaning of
Section 27A of the Securities Act of 1933 and Section 21E of the Securities
Exchange Act of 1934, including but not limited to statements regarding Summa's
expectations, hopes, beliefs, intentions or strategies regarding the future,
such as those set forth in Part II, Item 1 "Legal Proceedings" below. Actual
results could differ materially from those projected in any forward-looking
statements as a result of a number of factors, including those detailed in this
"Management's Discussion and Analysis" section (including, without limitation,
the potential material adverse consequences to the Company of the Year 2000
issue) and elsewhere herein and in the Company's Annual Report on Form 10-K for
the fiscal year ended August 31, 1998. The forward-looking statements are made
as of the date hereof, and the Company assumes no obligation to update the
forward-looking statements, or to update the reasons why actual results could
differ materially from those projected in the forward-looking statements.

        The Company designs and manufactures injection-molded plastic optical
components for OEM customers in the lighting industry; molded plastic modular
conveyor belt and chain for the food processing industry; engineered plastic
fittings, valves, filters and tubing for the agricultural irrigation industry;
and other molded and extruded plastic components for diverse industries. Growth
has been achieved by acquisition, development of new products and expansion of
the Company's sales organization. There can be no assurance that the Company
will be able to continue to consummate acquisitions, develop new products or
expand sales to sustain rates of revenue growth and profitability in future
periods comparable to those experienced in the past several years.

         Any future success that the Company may achieve will depend upon many
factors including factors which may be beyond the control of the Company or
which cannot be predicted at this time. These factors may include changes in the
markets for the products offered by the Company through its operating
subsidiaries, increased levels of competition including the entry of additional
competitors and increased success by existing competitors, reduced margins
caused by competitive pressures and other factors, increases in operating costs
including costs of production, supplies, personnel, equipment, import duties and
transportation, increases in governmental regulation imposed under federal,
state or local laws, including regulations applicable to environmental, labor
and trade matters, changing customer profiles and general economic and industry
conditions that affect customer demand and sales volume, both domestically and
in international markets, the introduction of new products by the Company or its
competitors, the need to make material capital expenditures, the timing of the
Company's advertising and promotional campaigns, and other factors.

                                       8



<PAGE>   9

RECENT EVENT
------------

        As previously reported by the Company in its current report on Form 8-K
filed March 17, 1999 (the "Form 8-K"), on March 5, 1999 the Company, through a
newly formed wholly-owned subsidiary, consummated its purchase of substantially
all of the assets of Plastron Industries L.P., a Delaware limited partnership
("Plastron"). Plastron, located in Bensenville, Illinois, is a leading
manufacturer of precision thermoplastic parts for wound electronic components
such as transformers, relays and coils, known in the industry as coil forms or
"bobbins." For the year ended December 31, 1998, Plastron reported sales of
$18.2 million. The aggregate purchase price paid for Plastron consisted of (i)
$20.0 million in cash, (ii) a four-year warrant exercisable to purchase up to
200,000 shares of the Company's common stock at $11.75 per share valued at
$278,000, (iii) investment banking fees consisting of a $125,000 cash payment
and 12,000 options to purchase the Company's stock at an average of the high and
low trading price of the Company's stock for the 20 days preceding the grant
valued at $33,000, and (iv) the assumption of certain liabilities, principally
trade payables and accrued obligations of approximately $1.0 million. The
purchase price is subject to a one time purchase price adjustment to be
calculated comparing Plastron working capital at closing to $1.85 million. The
transaction will be accounted for using the purchase method of accounting and,
accordingly, the purchase price has been allocated to identifiable tangible and
intangible assets purchased and liabilities assumed or incurred based upon their
fair value at the date of acquisition. The excess of the purchase price over the
fair values of the net assets acquired amounted to $13,803,000 and will be
recorded as goodwill which will be amortized on a straight line basis over 35
years. The funds used for the cash portion of the purchase price were borrowed
from the Company's primary lender pursuant to a revised credit facility. See
"--Liquidity and Capital Resources--Financing Arrangements" below. Plastron's
audited financial statements were previously filed in the Form 8-K. For pro
forma financial information relating to this acquisition, see Part II, Item 5
"Other Information--Acquisition" below.

        Following consummation of the acquisition, the Company sold to certain
management employees of Plastron 24,267 restricted shares of the Company's
common stock at a recent market price, and granted to certain employees of
Plastron non-statutory stock options to acquire 43,250 shares of the Company's
common stock at the same recent market price, which will vest based on the
percentage obtained by dividing the cumulative net income of Plastron after
closing by $3.0 million, or fully in nine years.

RESULTS OF OPERATIONS
---------------------

        The following table sets forth certain income information for the
Company's continuing operations as a percent of sales for the three-month and
six-month periods ended February 28, 1998 and 1999, and the Company's effective
income tax rate during those periods.

<TABLE>
<CAPTION>
                                            Three months ended February 28      Six months ended February 28
------------------------------------------------------------------------------------------------------------
                                                    1998             1999             1998             1999
------------------------------------------------------------------------------------------------------------
<S>                                                <C>              <C>              <C>              <C>
Net sales.......................................   100.0%           100.0%           100.0%           100.0%
Cost of sales...................................    70.8%            68.4%            70.0%            68.6%
                                                   -----            -----            -----            -----
Gross profit....................................    29.2%            31.6%            30.0%            31.4%
S,G & A and other expenses......................    19.7%            20.4%            20.2%            20.0%
                                                   -----            -----            -----            -----
Operating income from continuing operations.....     9.5%            11.2%             9.8%            11.4%
Interest expense, net...........................     2.2%             1.6%             1.8%             1.6%
                                                   -----            -----            -----            -----
Income from continuing operations before tax....     7.3%             9.6%             8.0%             9.8%
Provision for income taxes......................     3.0%             3.7%             3.3%             3.8%
                                                   -----            -----            -----            -----
Income from continuing operations...............     4.3%             5.9%             4.7%             6.0%
                                                   =====            =====            =====            =====
Effective tax rate..............................    40.5%            38.5%            41.1%            38.9%
</TABLE>



                                       9



<PAGE>   10

        Sales for the second quarter, ended February 28, 1999, increased
$2,577,000, or 13%, compared to the same period in the prior year due to
acquisitions and growth in the same business sales of 6%.

        Sales for the six months ended February 28, 1999 increased $9,414,000,
or 26%, primarily due to acquisitions and due to growth in the same business
sales of 3%.

        Gross profit for the second quarter increased $1,295,000, or 22%,
primarily due to growth and the effects of acquisitions. The gross profit
percentage increased from 29.2% to 31.6% as a result of the benefit of increased
volumes, the effect of blending recently acquired operations at typically higher
margins and cost reduction initiatives.

        Gross margins for the six months ended February 28, 1999 increased
$3,476,000, or 31%, from the comparable prior year period primarily due to the
effects of acquisitions. The gross profit percentage increased from 30.0% to
31.4% as a result of the one-time acquisition accounting effects in the prior
year first quarter, the benefit of increased volumes, cost reduction initiatives
and the benefit of blending recently acquired operations at typically higher
margins.

        Operating expenses for the second quarter increased $659,000, or 16%,
from the comparable prior year quarter primarily due to the inclusion of the
operating expenses of the recently required operations. As a percentage of
sales, operating expenses increased from 19.8% to 20.4%, primarily because
recently acquired operations typically operated with higher percentage operating
expenses. Operating margin increased from 9.5% to 11.2% as a result of the
changes in gross margin and operating expenses discussed above.

        Operating expenses for the six months ended February 28, 1999 increased
$1,832,000, or 25%, from the comparable prior year period primarily due to the
inclusion of the operating expenses of recently acquired operations, but as a
percentage of sales, decreased from 20.2% to 20.0%. Operating margin for the six
month period increased from 9.8% to 11.4% as a result of the changes in gross
margin and operating expenses discussed above.

        Net interest expense for the quarter ended February 28, 1999 decreased
$84,000 from the prior year second quarter due to decreased debt levels, a
decrease in the Company's weighted average interest rate and interest income on
the note received as partial consideration for the sale of GST Industries in
June 1998. For the six month period ended February 28, 1999, interest expense
increased $95,000 from the comparable prior year period due primarily to
increased debt levels related to acquisitions.

        The decrease in the effective tax rate in the current three month and
six month periods is primarily due to a lower effective combined state income
tax rate.

        The Company's backlog of unfilled orders, believed to be firm, decreased
slightly from $7,198,000 at August 31, 1998 to $7,055,000 at February 28, 1999.
Because the length of time between entering an order and shipping the product is
typically shorter than one month, backlog levels are not a reliable indicator of
future sales volume.

LIQUIDITY AND CAPITAL RESOURCES
-------------------------------

        Working Capital. The Company's working capital at February 28, 1999 was
$12,531,000 compared to $10,854,000 at August 31, 1998. The change was primarily
attributable to seasonal inventory growth and increased receivables related to
seasonal and other sales increases in the latter half of the second quarter.

        Financing Arrangements. The Company has several debt relationships in
place as described below. Substantially all of the Company's assets are pledged
to secure debt. The term debt and revolving line of credit require compliance
with various bank covenants.


                                       10



<PAGE>   11

        Summary of the Company's debt at February 28, 1999:

<TABLE>
<CAPTION>
                                                                          Weighted
                                                                           Average     Additional
               Description of Debt                         Balance   Interest Rate   Availability
               ----------------------------------------------------------------------------------
               <S>                                     <C>            <C>         <C>
               Bank term loan........................  $ 9,157,000            7.1%    $        --
               Bank line of credit...................    4,700,000            6.7%      9,338,000
               Bank acquisition line.................           --             --%      5,000,000
               Industrial revenue bonds and other....    5,928,000            6.3%        780,000
                                                       -----------            ---     -----------
               Total debt............................  $19,785,000            6.7%    $15,118,000
                                                       ===========            ===     ===========
</TABLE>

        In connection with the Company's acquisition of Plastron on March 5,
1999, (see "Recent Event" preceding), the Company entered into revised credit
facilities consisting of a new $12,000,000 term loan, an existing term loan with
a balance of $9,157,000 as of February 28, 1999 and a revolving line of credit
of up to $17,000,000 depending upon the amount of eligible collateral. The
revolving line of credit expires in December 2001 and the term loans mature in
2004.

        Summary of Company's debt at March 5, 1999:

<TABLE>
<CAPTION>
                                                                         Estimated
                                                        Balance at   Third Quarter     Additional
               Description of Debt                   March 5, 1999   Interest Rate   Availability
               ----------------------------------------------------------------------------------
               <S>                                   <C>             <C>             <C>
               Bank term loans.....................    $21,030,000            7.9%     $       --
               Bank line of credit.................     12,200,000            7.5%      1,800,000
               Industrial revenue bonds and other..      5,928,000            6.3%        780,000
                                                       -----------            ----     ----------
               Total debt..........................    $39,158,000            7.5%     $2,580,000
                                                       ===========            ====     ==========
</TABLE>

        In connection with the revised credit facilities, the margin over the
base rate charged to the Company by its primary lender was increased by
approximately 0.75%. Interest rates are subject to reduction as the Company
reduces its debt and achieves certain financial milestones.

        The Company announced a stock buy-back program September 28, 1998 which
authorized the Company to purchase its common stock in an aggregate amount of up
to $2,000,000. During the first quarter, the Company repurchased and retired
18,000 shares of its common stock in block trades, at an average price of $8.48
per share. There were no repurchases during the second quarter.

        Summa believes that cash flows from operations and existing credit
facilities will be sufficient to fund working capital requirements, planned
capital expenditures and debt service for the next twelve months. The Company
has a strategy of growth by acquisition. In the event an acquisition plan is
adopted which requires funds exceeding the availability described above, an
alternate source of funds to accomplish the acquisition would have to be
developed. The Company has 10,000,000 shares of common stock authorized, of
which 4,265,683 shares were outstanding at February 28, 1999 and 5,000,000
shares of "blank check" preferred stock authorized of which none is outstanding.
The Company could issue additional shares of common or preferred stock to raise
funds.




                                       11


<PAGE>   12

YEAR 2000 COMPLIANCE
--------------------

        The Company is continuing to analyze operations to determine and
implement the procedures necessary to ensure timely Year 2000 compliance. The
Company is also in the process of identifying and contacting key customers,
vendors and suppliers to request confirmation of timely external Year 2000
compliance.

        Each of the Company's facilities utilizes and is dependent upon data
processing systems and software to conduct business. The Company has received
confirmation from vendors of most of the business software used by the Company
that such software is designed to be Year 2000 compliant. Further, for reasons
generally unrelated to the Year 2000 issue, the Company is in the process of
purchasing and installing new systems for certain operations at a cost of
several hundred thousand dollars. The Company currently anticipates that all
internally used software will be Year 2000 compliant in a timely manner.
Additionally, various machines and other types of personal property at each
facility have computer controls and/or contain integrated circuits that may be
affected, and the Company is in the process of identifying and analyzing such
property to determine Year 2000 compliance.

        Although, the Company currently believes that it will be internally Year
2000 compliant in all material respects prior to January 1, 2000 and that the
effort to achieve Year 2000 compliance has not and will not have a significant
impact on the financial condition or results of future operations of the
Company, the Company remains concerned that the failure to comply by a
relatively small number of large customers and/or vendors, including banking
institutions, utilities, telecommunications and transportation companies, could
significantly disrupt operations at one or more of the Company's facilities. The
Company does not have a formalized Company-wide contingency plan covering worst
case scenarios in the event of Year 2000 non-compliance, but any such plan, if
and when formalized, would likely include technical contacts, access to backup
systems and alternative vendor sources, among other things. See the introductory
paragraph above in this "Management's Discussion and Analysis" section for
forward looking statements disclaimer.

PART II - OTHER INFORMATION

ITEM 1.  LEGAL PROCEEDINGS
--------------------------

        The Company encounters lawsuits from time to time in the ordinary course
of business and, at February 28, 1999, the Company or its affiliates were
parties to several civil lawsuits. Any losses that the Company may suffer from
current or future lawsuits, and the effect such litigation may have upon the
reputation and marketability of the Company's products, could have a material
adverse impact on the results of future operations, the financial condition and
prospects of the Company.

ITEM 2.  CHANGES IN SECURITIES
------------------------------

        None.

ITEM 3.  DEFAULT UPON SENIOR SECURITIES
---------------------------------------

        None.

ITEM 4.  SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS
------------------------------------------------------------

        At the Company's Annual Meeting of Stockholders held on December 10,
1998, the stockholders approved a proposal to adopt the Company's 1999 Stock
Option Plan under which options to purchase up to 500,000 shares of the
Company's Common Stock may be granted over a period of up to ten years.




                                       12


<PAGE>   13

        In addition, incumbent directors Coalson C. Morris, James R. Swartwout
and Byron C. Roth were reelected to the Board of Directors of the Company to
serve as one Class of the Board of Directors for a three year term and until
their successors are elected and qualified.

ITEM 5.   OTHER INFORMATION
---------------------------

ACQUISITION

               SUMMA AND PLASTRON PRO FORMA FINANCIAL INFORMATION

                                SUMMA INDUSTRIES
         UNAUDITED PRO FORMA CONDENSED CONSOLIDATED FINANCIAL STATEMENTS

The accompanying unaudited pro forma condensed consolidated financial statements
reflect the acquisition by a wholly-owned subsidiary of the Company of the
assets and operations of Plastron Industries L.P. The transaction will be
accounted as a purchase by the Company of the net assets of Plastron Industries
L.P.

The unaudited pro forma condensed consolidated balance sheet is based upon the
Company's unaudited consolidated balance sheet at February 28, 1999 and Plastron
Industries L.P's historical audited balance sheet as of December 31, 1998, and
is presented as if the transaction had been consummated on February 28, 1999.

The unaudited pro forma condensed consolidated statement of income for the year
ended August 31, 1998 gives effect to the purchase of the net assets and
operations of Plastron Industries L.P. as if the transaction had occurred at
September 1, 1997, the beginning of the Company's fiscal year ended August 31,
1998. The unaudited pro forma condensed consolidated statement of income
combines the audited income statement of the Company for the year ended August
31, 1998 and the audited income statement of Plastron Industries L.P. for the
year ended December 31, 1998, adjusted to conform to the Company's fiscal year
end.

The unaudited pro forma condensed consolidated statement of income for the six
month period ended February 28, 1999 gives effect to the purchase of the net
assets and operations of Plastron Industries L.P. as if the transaction had
occurred at September 1, 1998, the beginning of the six month period. The
unaudited pro forma condensed consolidated statement of income combines the
unaudited income statement of the Company for the six months ended February 28,
1999 and the unaudited income statement of Plastron Industries L.P., adjusted to
conform to the six month period ended February 28, 1999.

The pro forma adjustments are based upon available information and upon certain
assumptions which the management of the Company believes are reasonable.
However, the unaudited pro forma condensed consolidated financial statements do
not purport to be indicative of the results which would have been achieved if
the transaction had been completed on the respective dates above or of results
which may be achieved in the future.





                                       13

<PAGE>   14

                                SUMMA INDUSTRIES

           PRO FORMA CONDENSED CONSOLIDATED BALANCE SHEET (UNAUDITED)
                                February 28, 1999

<TABLE>
<CAPTION>

                                                                                     Pro forma
                                        Summa      Plastron    Adjustments            Combined
                                  -----------   -----------    -----------         -----------
<S>                               <C>           <C>            <C>                 <C>
ASSETS

Current assets:
   Cash and cash equivalents      $   324,000   $   270,000    $  (270,000)(1)     $   324,000
   Accounts receivable             13,745,000     1,887,000        (15,000)(2)      15,617,000
   Inventories                      9,964,000       947,000        146,000 (3)      11,057,000
   Prepaid expenses and other       1,376,000        26,000         37,000 (4)       1,439,000
                                  -----------   -----------    -----------         -----------
       Total current assets        25,409,000     3,130,000       (102,000)         28,437,000
                                  -----------   -----------    -----------         -----------
Property, plant and equipment      28,959,000     7,249,000     (2,350,000)(5)      33,858,000
   Less accumulated depreciation    8,923,000      (414,000)       414,000 (6)       8,923,000
   Net property, plant and
        equipment                  20,036,000     6,835,000     (1,936,000)         24,935,000
                                  -----------   -----------    -----------         -----------
Other assets                        1,016,000     1,025,000     (1,025,000)(7)       1,016,000
Goodwill and other intangibles     17,942,000            --     13,803,000 (8)      31,745,000
                                  -----------   -----------    -----------         -----------
        Total assets              $64,403,000   $10,990,000    $10,740,000         $86,133,000
                                  ===========   ===========    ===========         ===========


LIABILITIES AND STOCKHOLDERS' EQUITY

Current liabilities:
   Accounts payable               $ 5,083,000   $   905,000    $   410,000 (9)     $ 5,988,000
   Accrued liabilities              4,765,000       517,000       (412,000)(10)      5,280,000
   Current maturities of            3,030,000       550,000       (550,000)(11)      6,030,000
      long-term debt                                             3,000,000 (12)
   Due to seller                           --       446,000       (446,000)(10)             --
                                  -----------   -----------    -----------         -----------
    Total current liabilities      12,878,000     2,418,000      2,002,000          17,298,000
Long-term debt, net of current     16,755,000     4,400,000     (4,400,000)(11)     33,755,000
maturities                          3,857,000       250,000     17,000,000 (12)      3,857,000
Other long term liabilities                                       (250,000)(10)
                                  -----------   -----------    -----------         -----------
       Total liabilities           33,490,000     7,068,000     14,352,000          54,910,000
                                  -----------   -----------    -----------         -----------
Stockholders' equity

   Common stock                    18,541,000            --        310,000 (13)     18,851,000
   Retained earnings               12,372,000            --             --          12,372,000
   Partners' equity                        --     3,922,000     (3,922,000)(14)             --
                                  -----------   -----------    -----------         -----------
      Total stockholders' equity   30,913,000     3,922,000     (3,612,000)         31,223,000
                                  -----------   -----------    -----------         -----------
       Total liabilities and
           stockholders' equity   $64,403,000   $10,990,000    $10,740,000         $86,133,000
                                  ===========   ===========    ===========         ===========
</TABLE>


                                       14
<PAGE>   15


                                SUMMA INDUSTRIES

        PRO FORMA CONDENSED CONSOLIDATED STATEMENT OF INCOME (UNAUDITED)
                       For the Year Ended August 31, 1998

<TABLE>
<CAPTION>
                                 Historical    Historical      Pro forma            Pro forma
                                      Summa      Plastron    Adjustments             Combined
                                -----------   -----------    -----------         ------------
<S>                             <C>           <C>            <C>                 <C>
Net sales                       $85,704,000   $18,131,000     $       --         $103,835,000
                                -----------   -----------     ----------         ------------

Cost and expenses:
   Cost of sales                 59,197,000    13,865,000        267,000 (15)      73,329,000
   Selling and administrative
      and other operating
      expense                    17,127,000     2,190,000        394,000 (16)      18,777,000
                                                                (934,000)(17)
   Interest expense               1,607,000       552,000        851,000 (18)       3,010,000
                                -----------   -----------     ----------         ------------

       Total cost and expenses   77,931,000    16,607,000        578,000           95,116,000
                                -----------   -----------     ----------         ------------
Income from continuing
  operations before provision
  for taxes                       7,773,000     1,524,000       (578,000)           8,719,000


Provision for income taxes        3,215,000         2,000        367,000 (19)       3,584,000
                                -----------   -----------     ----------         ------------
Income from continuing
  operations                    $ 4,558,000   $ 1,522,000     $ (945,000)        $  5,135,000
                                ===========   ===========     ==========         ============
Income per common and
  equivalent share from
  continuing operations               $1.03                                             $1.16
                                ===========                                      ============
Weighted average shares
  outstanding - diluted           4,420,000                                         4,420,000
                                ===========                                      ============
</TABLE>

The pro forma condensed consolidated financial statements are adjusted as
follows:

        (1)  To eliminate cash retained by seller.
        (2)  Adjustment to allowance for doubtful accounts receivable.
        (3)  Fair market value adjustment of work in process and finished goods
             inventories to eliminate manufacturing profit and selling costs.
        (4)  To record deferred taxes.
        (5)  Adjustment of property, plant and equipment to estimated fair
             value.
        (6)  To reset accumulated depreciation of acquired assets to zero. 
        (7)  To delete other assets not acquired. 
        (8)  To record goodwill for the excess of purchase price over the fair
             value of net assets acquired.
        (9)  To accrue transaction fees and other costs.
        (10) To delete liabilities not acquired and to reflect adjustment to
             agreed working capital level.
        (11) To delete debt not acquired.
        (12) To reflect debt incurred in acquisition of Plastron. 
        (13) To record value assigned to warrants and stock options issued.
        (14) To delete former owner equity.
        (15) To charge cost of sales with write-up of inventory to fair value
             and increase in depreciation expense due to change to fair value
             and remaining lines.
        (16) To reflect amortization of goodwill created in purchase accounting,
             assuming useful life of 35 years.
        (17) To delete non-recurring and former owner expenses. 
        (18) To reflect increase in interest expense on net debt incurred.
        (19) To reflect tax at statutory rates on Plastron pre-tax income and
             tax effect on adjustments.




                                       15


<PAGE>   16

                                SUMMA INDUSTRIES

        PRO FORMA CONDENSED CONSOLIDATED STATEMENT OF INCOME (UNAUDITED)
                   For the Six Months Ended February 28, 1999

<TABLE>
<CAPTION>
                                       Historical      Historical         Pro forma           Pro forma
                                            Summa        Plastron       Adjustments            Combined
                                       ----------      ----------       -----------         -----------
<S>                                   <C>              <C>              <C>                 <C>        
Net sales                             $46,258,000      $8,574,000       $        --         $54,832,000
                                      -----------      ----------       -----------         -----------

Cost and expenses:
   Cost of sales                       31,732,000       6,305,000           227,000 (15)     38,264,000
   Selling and administrative and
      other operating expense           9,264,000       1,151,000           197,000 (16)     10,041,000
                                                                           (571,000)(17)
   Interest expense                       748,000         205,000           435,000 (18)      1,388,000
                                      -----------      ----------        ----------         -----------
       Total cost and expenses         41,744,000       7,661,000           288,000          49,693,000
                                      -----------      ----------        ----------         -----------
Income from continuing operations
   before provision for taxes           4,514,000         913,000         (288,000)           5,139,000


Provision for income taxes              1,755,000           8,000           236,000 (19)      1,999,000
                                      -----------      ----------        ----------         -----------
Income from continuing operations     $ 2,759,000      $  905,000        $ (524,000)        $ 3,140,000
                                      ===========      ==========        ==========         ===========
Income per common and equivalent
   share from continuing operations          $.62                                                  $.71
                                      ===========                                           ===========
 Weighted average shares
   outstanding - diluted                4,439,000                                                 4,439,000
                                      ===========                                               ===========
</TABLE>

        The pro forma condensed consolidated statement of income or the six
        month period ended February 28, 1999 is adjusted as follows:

        (15) To charge cost of sales will write-up of inventory of fair value
             and increase in depreciation expense due to change to
             fair value and remaining lives.
        (16) To reflect amortization of goodwill created in purchase accounting,
             assuming useful life of 35 years. 
        (17) To delete non-recurring and former owner expenses. 
        (18) To reflect increase in interest expense on net debt incurred. 
        (19) To reflect tax at statutory rates on Plastron pre-tax income and 
             tax effect on adjustments.







                                       16


<PAGE>   17

ENVIRONMENTAL
-------------

               Prior to October 1986, a previously owned business unit of one of
the Company's subsidiaries operated a facility on property within an area
subsequently designated as a federal Superfund site. The Company learned that
hazardous substances have been detected in the subsurface of the property and
that the current owner has been requested by a state agency to undertake
additional investigation at the property. The Company is also aware that the
property has been subject to a general notice letter issued by the United States
Environmental Protection Agency under the federal Superfund law. The Company, as
the successor to one of several prior tenants of the property, may be held
responsible for the contamination at the site regardless of whether its
subsidiary caused the contamination. The Company does not believe it is
responsible for any contamination at the property, and has not been notified or
contacted by any governmental authority in that regard, nor named in any
proceeding relating to the property. However, if the Company were held liable
under federal Superfund law, or other environmental law, or had to defend itself
against such a claim, the consequences could be material to the Company's
financial statements.

ITEM 6.  EXHIBITS AND REPORTS ON FORM 8-K
-----------------------------------------

           (A) EXHIBITS.

               27.1   Financial Data Schedule

           (B) CURRENT REPORTS ON FORM 8-K.

               Subsequent to the end of the fiscal quarter, the Company filed a
current report on Form 8-K on March 17, 1999.



                                   SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the
Registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized on April 2, 1999.

                                SUMMA INDUSTRIES

/s/ James R. Swartwout                             /s/ Trygve M. Thoresen
-------------------------------------              -----------------------------
James R. Swartwout                                 Trygve M. Thoresen
President and Chief Financial Officer              Vice President and Secretary






                                       17


<PAGE>   18

                                 EXHIBIT INDEX

              Exhibit
              Number               Description
              -------              -----------

               27.1                Financial Data Schedule