CHECKING CLEAR COAT THICKNESS BEFORE & AFTER PAINT FINESSE

SUBJECT: IMPORTANCE OF CHECKING CLEAR COAT THICKNESS BEFORE AND AFTER PAINT FINESSE OPERATIONS

MODELS: 1984 AND FUTURE TRUCKS WITH CLEAR COAT PAINT FILM

Finesse procedures ("Removal of Chemical Spotting and Other Minor Paint Imperfections Without Repainting") 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.

GMC Truck recommends a paint thickness gage 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 ultra- violet 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).

GMC Truck recommends the digital Elcometer 300 fn paint thickness gage, or equivalent, for its 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, GMC Truck recommends the digital Elcometer 246 model, or equivalent. 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. 1893 Rochester Ind. Dr. 410 Cedar Street Rochester Hills, MI 48309 Ogdensburg, NY 13669 (800) 521-0635 or (800) 267-0607 or (313) 651-0500 (613) 925-5987

Zorelco, Ltd/Zormco 8520 Garfield Road Cleveland, OH 44125 (216) 441-6102

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.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.