I've been using an old paint thickness gauge for decades. You can still find them if you know what you're looking for.
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Paint Thickness Gauge,Paint Coating Tester, Car Body Damage Detector
Buy one of these from Amazon. They're only $14.88
Paint Thickness Gauge,Paint Coating Tester, Car Body Damage Detector,Crash-Test Check, Car Paint Inspection,Water Resistant, Great to Have When Shopping for Used car(Size:17.8x7.2x1.7cm)
- This is a magnetic paint thickness gauge. It’s magnetic tip is extremely convenient in reaching curved surfaces, and in hard to reach places.Its mechanical thickness gauge is water resistant, no batteries needed and can be used in the rain and on a dirty car.
- Colored gauge shows you exactly what you're dealing with as to thickness of metals and the paint on it, and it will easily reveal any damages.
- The tester instrument is highly accurate with a micron-level color scale due to its compatibility with digital thickness gauge. Quickly and accurately check if the vehicle has been painted in the past.
- It’s shaped as a small pen, and is made of high quality materials, making it the perfect choice to carry around and always be prepared.
- Package includes:1* Paint thickness tester
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Here's a pair of pictures of the one I've used for years.
You place the end against the paint and a ring on the end of the clear tube surrounds a magnet in the black portion of the tool.
The magnet "sticks" to the body. You, then, pull on the black plastic and the magnet and spring-loaded plunger stays in place still stuck to the body while the black, plastic, sleeve surrounding the magnet pulls away from the body.
Because the clear tube has a lip on the bottom, it stays in place along with the spring-loaded plunger with the magnet and the rear end of the clear plastic tube creeps down the scale at the back end as you pull back on the tool.
When the internal spring pressure overcomes the magnet, the whole tool comes loose and you, then, look at the scale and it reads the mil thickness of the paint.
While it was designed to measure paint thickness so that you can tell if the panel has been re-painted, it will also give you a quick indication if there's any body filler under the paint.
I've saved customers a lot of money over the years when checking out "professional" restorations that were actually whored up auction specials.
From experience, never go to an auction, especially a Barret Jackson auction, without one of these.