The Paint Thickness Tester is a cheap keychain device that promises to help you avoid bringing home a lemon from the lot of your local sleazy used car Guido by telling you when a car has well-hidden touch-ups in its paint job:
Place the test probe on the car roof, for example. By pressing a button, the paint thickness tester will store this paint thickness as a reference value. Now, by placing the test probe on any part of the vehicle body, you can compare it with this value. You will immediately determine whether parts of the body have been replaces or repainted and if the purchase price is suitable.
I've never owned a car, so maybe someone can fill me in: if a paint job has been touched up skillfully and it's not visible to the naked eye, why does it matter if it isn't all the original coat? It seems pointless to me. Still, I guess $20 is a cheap enough price to give you some empirical proof of your used car not being worth the sticker price when you're brought into the negotiating room.
Update: Uh. Yeah. Duh. Peter S. Conrad explains why this is important in a way that makes sense even to a committed, non-license-carrying pedestrian like myself: "Well, for example, a car that has the same thickness paint all over has probably never been in a crash, probably doesn't have big rust holes filled in with Bondo, etc."
Paint Thickness Tester [Official Site via OhGizmo!]

























