Originally Posted by Range Rover

Very valid point, and that's where it becomes a bit of a "grey area." It's very possible that many of the cars I've metered had paintwork done at the factory. On small-scale production cars like the 928, I imagine factory paint rework would be common - especially on special orders (PTS, rarely selected colors, etc.). In fact, I was recently at a Porsche dealer looking at a brand new black 992 and there was very clearly a factory flaw (ripple) in the original paint on the passenger door. I didn't take a meter to the car, but wouldn't be surprised if there was some variation or if the factory repainted that panel. It would be hard if not impossible to know if the factory repainted a panel because, as far as I know, there would be no record of it.

With regards to measurements, I tend to look at anything exceeding 8 mils as being repainted, but that's just my own de facto metric. Basically, I never take anyone's word for it regarding the originality of the paint, and as these cars become older and as documentation gets lost, it becomes very difficult to prove originality.