Paint - Importance of Checking Clear Coat Thickness
9216chevy08
Number: 91-282-10
Section: 10
Date: APRIL 1991
Corporate Bulletin No.: 111701R
Subject:
IMPORTANCE OF CHECKING CLEAR COAT
THICKNESS BEFORE AND AFTER PAINT
FINESSE OPERATIONS
Model and Year:
1984-91 ALL PASSENGER CARS
1984-91 M VAN AND C/K TRUCKS
1987-9.1 R/V TRUCKS
1990-91 L AND U VAN
THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 91-255-10, DATED MARCH 1991. THE SUPPLIER ADDRESS HAS BEEN CORRECTED. ALL COPIES OF 91-255-10 SHOULD BE DISCARDED.
Finesse procedures may result in a lowering of the clear coat protective film below the level necessary to provide minimum durability and performance requirements. As time progresses, a severe "whitening" or degradation of the clear coat may occur when the clear coat is removed below the minimum GM specification.
A paint thickness gage should be used to determine how much of the clear coat has been removed by the polishing or compounding process. Readings of the paint film build should be taken prior to the finesse operation. As the finesse operation progresses, additional readings should be taken to determine the impact on the total film build. If the gage indicates the total thickness has been reduced by .5 mils, it will be necessary to re-spray clear on the repair area to restore the needed ultraviolet protection. If the base coat is disturbed, a complete repainting process is required.
There are many types of paint thickness gages available to the refinisher in today's market. The older magnetic-type gages, the best of which have a +/- 5% accuracy range, are not sensitive enough to detect a .5 mil removal of clear coat. Most gages are confined to checking either a ferrous substrate (steel), or a non-ferrous metal substrate. At this time, there is no viable gage for reading film thicknesses on a non-metallic substrate (such as plastic or rubber).
The digital Elcometer 300 fn paint thickness gage, or equivalent, has the ability to read film thicknesses on both ferrous and non-ferrous metal substrates. With thickness standards included for recalibration purposes, this instrument is capable of reading film thicknesses within an accuracy range of +/- 1%. The approximate cost of the unit is $1750.
As an alternative unit for the refinisher who deals only with traditional steel components. The digital Elcometer 246 model, or equivalent may be used. The gage demonstrates a +/- 3% accuracy range and comes with thickness standards for recalibration. The approximate cost of this unit is $575.
Other digital paint thickness gages are available from the suppliers listed below which demonstrate the same accuracy levels as the recommended Elcometer models:
Elcometer, Inc. Defelsko Corp. Zelcro, Ltd./Zormco
1893 Rochesterlnd. Drive 410 Cedar Street 8520 Garfield Road
Rochester Hills, MI 48309 Ogdensburg, NY 13669 Cleveland, OH 44125
(800) 521-0635 or (800) 267-0607 or (216) 441-6102
(313) 650-0500 (613) 925-5987
We believe these sources, their equipment, and materials to be reliable. There may be additional manufacturers of such materials and equipment. General Motors does not endorse, indicate any preference for, or assume responsibility for the products or equipment from these firms or for any such items which may be available from other sources.