<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
                                  MAY 31, 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
reports), 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 May 31, 1999 was
4,299,826.



<PAGE>   2

                                SUMMA INDUSTRIES

                                      INDEX


<TABLE>
<CAPTION>
         PART I - FINANCIAL INFORMATION                                                                                  Page
         <S>                                                                                                             <C>
         Item 1.  Financial Statements:

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

                  Condensed Consolidated Statements of Income (unaudited) -
                  three months and nine months ended May 31, 1998 and 1999................................................4

                  Condensed Consolidated Statements of Cash Flows (unaudited) -
                  nine months ended May 31, 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 2.  Changes in Securities..................................................................................12
         Item 3.  Default upon Senior Securities.........................................................................12
         Item 4.  Submission of Matters to a Vote of Security Holders....................................................12
         Item 5.  Other Information......................................................................................12
         Item 6.  Exhibits and Reports on Form 8-K.......................................................................13

         Signature Page..................................................................................................13

</TABLE>




                                       2


<PAGE>   3
                                SUMMA INDUSTRIES
                      CONDENSED CONSOLIDATED BALANCE SHEETS
<TABLE>
<CAPTION>

                                                                                         August 31, 1998           May 31, 1999
 ASSETS                                                                                                             (unaudited)
 ------------------------------------------------------------------------------------------------------------------------------
<S>                                                                                             <C>                    <C>
 Current assets:
     Cash and cash equivalents                                                                  $293,000               $291,000
     Accounts receivable                                                                      12,975,000             17,020,000
     Inventories                                                                               9,392,000             11,607,000
     Prepaid expenses and other                                                                1,439,000              1,385,000
 ------------------------------------------------------------------------------------------------------------------------------
        Total current assets                                                                  24,099,000             30,303,000
 ------------------------------------------------------------------------------------------------------------------------------
 Property, plant and equipment                                                                27,796,000             34,596,000
    Less accumulated depreciation                                                              7,132,000              9,964,000
 ------------------------------------------------------------------------------------------------------------------------------
         Net property, plant and equipment                                                    20,664,000             24,632,000
 ------------------------------------------------------------------------------------------------------------------------------
 Other assets                                                                                  1,006,000              1,022,000
 Goodwill and other intangibles, net                                                          18,214,000             31,968,000
 ------------------------------------------------------------------------------------------------------------------------------
       Total assets                                                                          $63,983,000            $87,925,000
 ==============================================================================================================================
 LIABILITIES AND STOCKHOLDERS' EQUITY
 ------------------------------------------------------------------------------------------------------------------------------
 Current liabilities:
    Accounts payable                                                                          $5,299,000             $6,669,000
    Accrued liabilities                                                                        5,279,000              6,624,000
    Current maturities of long-term debt                                                       2,667,000              6,129,000
 ------------------------------------------------------------------------------------------------------------------------------
       Total current liabilities                                                              13,245,000             19,422,000
 ------------------------------------------------------------------------------------------------------------------------------
 Long-term debt, net of current maturities                                                    18,675,000             31,195,000
 Other long-term liabilities                                                                   3,945,000              3,905,000
 ------------------------------------------------------------------------------------------------------------------------------
       Total liabilities                                                                      35,865,000             54,522,000
 ------------------------------------------------------------------------------------------------------------------------------
 Stockholders' equity:
    Common stock, par value $.001; 10,000,000 shares authorized;
          issued and outstanding: 4,257,307 at August 31, 1998 and
          4,299,826 at May 31, 1999                                                           18,505,000             19,151,000
    Retained earnings                                                                          9,613,000             14,252,000
 ------------------------------------------------------------------------------------------------------------------------------
       Total stockholders' equity                                                             28,118,000             33,403,000
 ------------------------------------------------------------------------------------------------------------------------------
       Total liabilities and stockholders' equity                                            $63,983,000            $87,925,000
 ==============================================================================================================================
</TABLE>

See accompanying notes to condensed consolidated financial statements.


                                       3

<PAGE>   4
                                SUMMA INDUSTRIES
                   CONDENSED CONSOLIDATED STATEMENTS OF INCOME
                                   (unaudited)
<TABLE>
<CAPTION>
                                                              Three months ended May 31               Nine months ended May 31
------------------------------------------------------------------------------------------------------------------------------
                                                              1998                 1999                 1998              1999
------------------------------------------------------------------------------------------------------------------------------
<S>                                                    <C>                  <C>                  <C>               <C>
Net sales                                              $23,854,000          $30,203,000          $60,698,000       $76,461,000
Cost of sales                                           16,377,000           20,974,000           42,121,000        52,706,000
------------------------------------------------------------------------------------------------------------------------------
Gross profit                                             7,477,000            9,229,000           18,577,000        23,755,000
Selling, general, administrative and other
    expenses                                             4,649,000            5,394,000           12,131,000        14,658,000
------------------------------------------------------------------------------------------------------------------------------
Operating income from continuing operations              2,828,000            3,835,000            6,446,000         9,097,000
Interest expense                                           506,000              798,000            1,159,000         1,546,000
------------------------------------------------------------------------------------------------------------------------------
Income from continuing operations before
    provision for taxes                                  2,322,000            3,037,000            5,287,000         7,551,000
Provision for income taxes                                 988,000            1,157,000            2,207,000         2,912,000
------------------------------------------------------------------------------------------------------------------------------
Income from continuing operations                        1,334,000            1,880,000            3,080,000         4,639,000
Income from discontinued operations, net of the
   effect of income tax                                     66,000                  ---              299,000               ---
------------------------------------------------------------------------------------------------------------------------------
Net income                                              $1,400,000           $1,880,000           $3,379,000        $4,639,000
==============================================================================================================================
Earnings per common share
------------------------------------------------------------------------------------------------------------------------------
Basic
       Continuing operations                                  $.32                 $.44                 $.74             $1.09
       Discontinued operations                                $.01                 $---                 $.07             $ ---
       Net income                                             $.33                 $.44                 $.81             $1.09

==============================================================================================================================
Diluted
       Continuing operations                                  $.30                 $.42                 $.70             $1.04
       Discontinued operations                                $.01                 $---                 $.07             $ ---
       Net income                                             $.31                 $.42                 $.77             $1.04
==============================================================================================================================
Weighted average common shares outstanding
       Basic                                             4,244,000            4,289,000            4,181,000         4,269,000
       Diluted                                           4,502,000            4,481,000            4,406,000         4,453,000
------------------------------------------------------------------------------------------------------------------------------
</TABLE>

See accompanying notes to condensed consolidated financial statements.



                                       4

<PAGE>   5
                                SUMMA INDUSTRIES
                 CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
                                   (unaudited)

<TABLE>
<CAPTION>

                                                                                                      Nine months ended May 31
 -----------------------------------------------------------------------------------------------------------------------------
                                                                                                    1998                  1999
 -----------------------------------------------------------------------------------------------------------------------------
<S>                                                                                           <C>                   <C>
 Operating activities:
 Net income                                                                                   $3,379,000            $4,639,000
 -----------------------------------------------------------------------------------------------------------------------------
 Adjustments to reconcile net income to net cash provided by operating
   activities:
     Depreciation                                                                              2,357,000             2,881,000
     Amortization                                                                                363,000               507,000
     Loss (gain) on disposition of property, plant and equipment                                 117,000               (12,000)
     Net change in assets and liabilities, net of effects of
       acquisitions:                                                                          (1,591,000)           (1,987,000)
        Accounts receivable                                                                     (474,000)             (895,000)
        Inventories                                                                               73,000               146,000
        Prepaid expenses and other assets                                                        130,000              (222,000)
        Accounts payable                                                                         350,000               691,000
        Accrued liabilities
 -----------------------------------------------------------------------------------------------------------------------------
           Total adjustments                                                                   1,325,000             1,109,000
 -----------------------------------------------------------------------------------------------------------------------------
          Net cash provided by operating activities                                            4,704,000             5,748,000
 -----------------------------------------------------------------------------------------------------------------------------
 Investing activities:
 Acquisition of businesses (Note 5)                                                          (22,859,000)          (20,130,000)
 Purchases of property and equipment                                                          (2,072,000)           (1,953,000)
 Proceeds from sale of equipment                                                                   6,000                15,000
 Net decrease in unexpended revenue bond proceeds                                                371,000                   ---
 -----------------------------------------------------------------------------------------------------------------------------
          Net cash (used in) investing activities                                            (24,554,000)          (22,068,000)
 -----------------------------------------------------------------------------------------------------------------------------
 Financing activities:
 Net proceeds from line of credit                                                              6,865,000             7,908,000
 Proceeds from issuance of long-term debt                                                     13,500,000            13,227,000
 Payments on long-term debt                                                                   (4,380,000)           (5,153,000)
 Proceeds from the exercise of stock options                                                     864,000               489,000
 Purchases of common stock                                                                           ---              (153,000)
 -----------------------------------------------------------------------------------------------------------------------------
          Net cash provided by financing activities                                           16,849,000            16,318,000
 -----------------------------------------------------------------------------------------------------------------------------
 Net decrease in cash and cash equivalents                                                    (3,001,000)               (2,000)
 Cash and cash equivalents, beginning of period                                                3,020,000               293,000
 -----------------------------------------------------------------------------------------------------------------------------
 Cash and cash equivalents, end of period                                                        $19,000              $291,000
 =============================================================================================================================
</TABLE>


See accompanying notes to condensed 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 condensed 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 nine months ended May 31, 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             May 31, 1999
                                        ---------------             ------------

<S>                                         <C>                      <C>
Finished goods ...............              $ 3,611,000              $ 4,816,000
Work in process ..............                  111,000                  261,000
Materials and parts ..........                5,670,000                6,530,000
                                            -----------              -----------
                                            $ 9,392,000              $11,607,000
                                            ===========              ===========
</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 May 31              Nine months ended May 31
                                                            1998               1999               1998               1999
                                                            ----               ----               ----               ----
<S>                                                    <C>                <C>                <C>                <C>
Weighted average shares outstanding - basic .          4,244,000          4,289,000          4,181,000          4,269,000
Effect of dilutive securities:
   Impact of common shares to be issued under
      stock option plans ....................            258,000            192,000            225,000            184,000
                                                       ---------          ---------          ---------          ---------
Weighted average shares outstanding - diluted          4,502,000          4,481,000          4,406,000          4,453,000
                                                       =========          =========          =========          =========
</TABLE>



                                       6

<PAGE>   7

4.   SUPPLEMENTAL CASH FLOW INFORMATION

<TABLE>
<CAPTION>
                                                                 Nine months ended May 31
                                                              1998                   1999
                                                              ----                   ----
<S>                                                   <C>                    <C>
Cash paid during the period:
   Interest ................................          $  1,198,000           $  1,419,000
                                                      ============           ============
   Income taxes ............................          $  2,052,000           $  2,918,000
                                                      ============           ============
Non-cash investing and financing activities:
Details of acquisitions
   Fair value of assets acquired ...........          $ 37,317,000           $ 22,660,000
   Liabilities assumed or incurred .........           (11,707,000)            (2,220,000)
   Value of options and warrants issued ....            (1,345,000)              (310,000)
                                                      ------------           ------------
Cash paid ..................................            24,265,000             20,130,000
Less cash acquired .........................            (1,406,000)                   ---
                                                      ------------           ------------
Net cash used in acquisitions ..............          $ 22,859,000           $ 20,130,000
                                                      ============           ============
</TABLE>

5.       ACQUISITIONS

         On March 5, 1999, the Company completed the acquisition of
substantially all of the assets of Plastron Industries, L.P. ("Plastron"). 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 stock options,
valued at $33,000; and (iv) the assumption of certain liabilities, principally
trade payables and accrued obligations of approximately $2.2 million. The
purchase price is subject to a one time purchase price reduction to be
calculated comparing Plastron working capital at closing to $1.85 million. The
transaction 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 values of the net assets acquired amounted to $14,261,000 and has been
recorded as goodwill which is being amortized on a straight line basis over 35
years.

         On May 1, 1998, the Company completed the acquisition of Falcon
Belting, Inc. ("Falcon") of Oklahoma City, Oklahoma. The total acquisition cost
was $5,125,000, consisting of $2,636,000 in cash and the present value of
obligations to make future payments to the former owner of Falcon and
liabilities assumed or incurred of $2,489,000. The acquisition was 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 $1,995,000 and has been recorded as goodwill which
is being amortized on a straight-line basis over 30 years.


                                       7


<PAGE>   8

         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 stock 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, Falcon and Plastron have been
included in the consolidated results of operations and the consolidated
statements of cash flows of the Company since October 28, 1997; May 1, 1998 and
March 5, 1999, the respective dates of the acquisitions. The following unaudited
pro forma financial information presents the results of operations of the
continuing businesses of the Company with Calnetics and Plastron as if they had
been acquired as of September 1, 1997. Pro forma adjustments have been made to
give 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 acquisitions. The following unaudited pro forma financial information does
not include adjustments to give effect to the Falcon acquisition as such
adjustments would not be material.

<TABLE>
<CAPTION>
                                                                Three months ended May 31            Nine months ended May 31
                                                                     1998            1999               1998             1999
                                                                     ----            ----               ----             ----
<S>                                                           <C>             <C>                <C>              <C>
Net sales ..........................................          $28,160,000     $30,203,000        $80,015,000      $85,527,000
Income from continuing operations ..................            1,398,000       1,880,000          3,887,000        4,962,000
 Income from continuing operations per common share:
              basic ................................                 $.33            $.44               $.86            $1.16
              diluted ..............................                 $.31            $.42               $.81            $1.11
</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 acquisitions 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.



                                       8



<PAGE>   9
         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;
thermoplastic coil forms for electrical device manufacturers; 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.

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
nine-month periods ended May 31, 1998 and 1999, and the Company's effective
income tax rate during those periods.

<TABLE>
<CAPTION>
                                                  Three months ended May 31        Nine months ended May 31
                                                       1998            1999            1998            1999
                                                       ----            ----            ----            ----
<S>                                                   <C>             <C>             <C>             <C>
Net sales ..................................          100.0%          100.0%          100.0%          100.0%
Cost of sales ..............................           68.7%           69.4%           69.4%           68.9%
                                                      -----           -----           -----           -----
Gross profit ...............................           31.3%           30.6%           30.6%           31.1%
S,G & A and other expenses .................           19.5%           17.9%           20.0%           19.2%
                                                      -----           -----           -----           -----
Operating income from continuing operations            11.8%           12.7%           10.6%           11.9%
Interest expense, net ......................            2.1%            2.6%            1.9%            2.0%
                                                      -----           -----           -----           -----
Income from continuing operations before tax            9.7%           10.0%            8.7%            9.9%
Provision for income taxes .................            4.1%            3.8%            3.6%            3.8%
                                                      -----           -----           -----           -----
Income from continuing operations ..........            5.6%            6.2%            5.1%            6.1%
                                                      =====           =====           =====           =====
Effective tax rate .........................           42.6%           38.1%           41.7%           38.6%
</TABLE>

         Sales for the third quarter ended May 31, 1999 increased $6,349,000, or
27%, compared to the same period in the prior year due to acquisitions and
growth in the same business sales of 4%.

         Sales for the nine months ended May 31, 1999 increased $15,763,000, or
26%, from the comparable prior year period primarily due to acquisitions and due
to growth in the same business sales of 3%.



                                       9


<PAGE>   10

         Gross profit for the third quarter increased $4,597,000, or 28%, from
the comparable prior year period, primarily due to the effects of acquisitions
and sales growth. As a percentage of sales, gross profit decreased from 31.3% to
30.6%, as a result of the blending of newly acquired businesses with
historically lower gross margins and a one-time charge to cost of sales of
$148,000, or 0.5% of sales, for the write-up of inventories of acquired
operations, partially offset by the benefit of increased volume and cost
reduction initiatives.

         Gross profit for the nine months ended May 31, 1999 increased
$10,585,000, or 25%, from the comparable prior year period, primarily due to the
effects of acquisitions. As a percentage of sales, gross profit increased from
30.6% to 31.1%, primarily as a result of the benefit of increased volumes, and
cost reduction initiatives.

         Operating expenses for the third quarter increased $745,000, or 16%,
from the comparable prior year quarter, primarily due to the inclusion of the
operating expenses of recently acquired businesses. As a percentage of sales,
operating expenses decreased from 19.5% to 17.9%, because of the blending of
newly acquired businesses with historically lower operating expenses and the
benefit of increased volumes. Operating margin for the third quarter increased
from 11.8% to 12.7%, as a result of the changes in gross margin and operating
expenses discussed above.

         Operating expenses for the nine months ended May 31, 1999 increased
$2,527,000, or 21%, from the comparable prior year period, primarily due to the
inclusion of the operating expenses of recently acquired businesses. As a
percentage of sales, operating expenses decreased from 20.0% to 19.2% primarily
as a result of the benefit of increased volumes. Operating margin for the nine
month period increased from 10.6% to 11.9%, as a result of the changes in gross
margin and operating expenses discussed above.

         Net interest expense for the third quarter ended May 31, 1999 increased
$292,000 from the prior year third quarter, primarily due to increased debt
levels. For the nine month period ended May 31, 1999, interest expense increased
$387,000 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
nine month periods is due to a lower effective combined state income tax rate
and increased foreign sales corporation tax benefit.

         The Company's backlog of unfilled orders, believed to be firm,
increased from $7,198,000 at August 31, 1998 to $9,813,000 at May 31, 1999 due
to the inclusion of the backlog of acquired operations and increased bookings.
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.




                                       10


<PAGE>   11
LIQUIDITY AND CAPITAL RESOURCES

         Working Capital. The Company's working capital at May 31, 1999 was
$10,881,000 compared to $10,854,000 at August 31, 1998. Increases in accounts
receivable and inventory during the third quarter, primarily due to an
acquisition, were offset by increases in the current portion of debt and other
current liabilities.

         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.

Summary of the Company's debt at May 31, 1999:

<TABLE>
<CAPTION>
                                                            Weighted Average      Additional
Description of Debt                             Balance        Interest Rate    Availability             Due
-------------------                             -------     ----------------    ------------             ---
<S>                                         <C>                         <C>       <C>              <C>
Bank line of credit ..............          $10,642,000                 7.4%      $6,358,000            2001
Bank term loans ..................           20,276,000                 7.7%             ---       1999-2004
Industrial revenue bonds and other            6,406,000                 6.4%         780,000       1999-2021
                                            -----------                 ---       ----------
Total debt .......................          $37,324,000                 7.4%      $7,138,000
                                            ===========                 ===       ==========
</TABLE>

         Interest rates on bank debt are subject to reduction as the Company
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 or third quarters.

         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 4,299,826 shares were outstanding at May 31, 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 or enter into
new or revised borrowing arrangements to raise funds.

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


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<PAGE>   12

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 May 31, 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

         On March 31, 1999, following consummation of the acquisition of assets
of Plastron Industries, L.P., the Company sold 24,267 restricted shares of the
Company's common stock to certain management employees of Plastron, at the then
current market price, for a total consideration of $231,800, and granted
non-qualified stock options to certain Plastron employees, at the then current
market price. The options will vest based on the percentage obtained by dividing
the cumulative net income of Plastron after March 5, 1999 by $3.0 million, or
fully in nine years.

ITEM 3.  DEFAULT UPON SENIOR SECURITIES

         None.

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

         None.

ITEM 5.  OTHER INFORMATION

         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


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<PAGE>   13

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.

             4.1  Warrant dated March 5, 1999 issued by the Company to Plastron
                  Industries, L.P., providing for the issuance from time to time
                  of up to 200,000 shares of the Company's common stock at an
                  exercise price of $11.75 per share, including registration
                  rights.*

            10.1  Amended and Restated Loan Agreement dated March 5, 1999
                  between the Company and Comerica Bank - California. *

            10.2  Plastron Industries Acquisition Stock Option Plan providing
                  for the issuance of options to purchase up to 200,000 shares
                  of the Company's common stock.*

            27.1  Financial Data Schedule

             --------------------
             *    Incorporated by reference from the Company's Form 8-K dated
                  March 5, 1999 relating to the acquisition of Plastron
                  Industries, L.P.

         (b) CURRENT REPORTS ON FORM 8-K.

             Form 8-K dated March 5, 1999 relating to the acquisition of
Plastron Industries, L.P. by the Company.



                                       13

<PAGE>   14

                                   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 June 18, 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






                                       14

<PAGE>   15
                                 EXHIBIT INDEX
                                 -------------

   EXHIBIT
     NO.                      DESCRIPTION
   -------                    -----------

     4.1       Warrant dated March 5, 1999 issued by the Company to Plastron
               Industries, L.P., providing for the issuance from time to time of
               up to 200,000 shares of the Company's common stock at an exercise
               price of $11.75 per share, including registration rights.*

    10.1       Amended and Restated Loan Agreement dated March 5, 1999 between
               the Company and Comerica Bank - California. *

    10.2       Plastron Industries Acquisition Stock Option Plan providing for
               the issuance of options to purchase up to 200,000 shares of the
               Company's common stock.*

    27.1       Financial Data Schedule

--------------------
*    Incorporated by reference from the Company's Form 8-K dated
     March 5, 1999 relating to the acquisition of Plastron
     Industries, L.P.