How does a paint depth gauge measure paint?
By sensing the panel through the paint, with the method chosen by what the panel is made of. A probe rests on the surface, and the gauge works out how far away the metal or the next material sits.
| Panel material | Gauge type | Standard |
|---|---|---|
| Steel | Magnetic | ASTM D7091 |
| Aluminium | Eddy current | ASTM D7091 |
| Plastic or fibre composite, such as a plastic bumper | Ultrasonic | ASTM D6132 |
ASTM D7091 covers magnetic gauges for non-magnetic coatings over ferrous metal and eddy-current gauges for non-conductive coatings over non-ferrous metal, and it says the gauges measure coating layers once they’re dry. ASTM D6132 covers ultrasonic gauges on substrates including plastic and fibre composites, and notes that some can pick out individual layers in a multi-layer system.
What do the numbers mean?
They tell you how much paint a panel carries, which matters because the clear coat on top is thin. Readings come in microns or mils; one mil is 25.4 µm.
By Chrysler’s count in bulletin 31-001-13, 1.5 mils of clear coat is typical, and GM’s PIT5763 gives 1.5 to 2 mils for the 2021 Tahoe and Yukon. Chrysler’s technicians take a gauge reading before a repair and another after it, which shows exactly how much came off. A panel reading well above or below its neighbours deserves a question about its history before anyone polishes it.
Why take a reading on a used BC car?
Because a used car’s paint history doesn’t show to the eye. TranBC ties winter highway abrasives to windshield chips and paint dings, so a car with years of BC driving may have had chipped panels refinished, or been polished more than once. A reading tells the shop how far correction can safely go, and it matters for film as well. In its PPF FAQ, LLumar describes its film as made for original paint and excludes trouble with repainted or aftermarket paint from its warranty. Buying film for a second-hand car is covered in PPF on a used car.
Where paint readings come up at Brookswood Tint
Ahead of every polisher. Readings come first at the shop, and they decide the lightest correction level that will do the job; coating packages with correction work the same way. What each level spends in clear coat is on the clear coat cost of correction, and the levels themselves are on paint correction.