What can a paint-thickness gauge tell you about a used car?
A paint-thickness gauge measures the total thickness of everything on top of the metal: primer, base coat, clear coat — and filler, if any. Used panel by panel and compared, it shows which panels are likely original and which have been repainted or repaired.
How it works
On steel, gauges use magnetic induction: the probe measures how strongly it is attracted to the steel below, which decreases with distance. On aluminium they use eddy currents induced in the non-ferrous metal. Most professional gauges switch automatically between the two. Plastic and carbon panels cannot be measured this way; they need ultrasonic gauges.
What readings mean
Factory paint on modern cars is commonly in the region of 80–160 µm in total, and fairly consistent across panels of the same car. A repaint adds its own layers, so readings rise. Readings far above that range usually indicate body filler underneath. Low readings, close to the bottom of the range or below it, can mean an over-polished clear coat with little margin left.
The pattern matters more than any single number: one door reading 40–60% higher than the others, or readings that vary strongly within one panel, tell more than an absolute value.
How to measure properly
Calibrate on the zero plates supplied. Take several readings per panel (centre and edges), compare left and right, and record them. Avoid edges and curves for the main readings. Remember that some cars receive factory touch-ups or repaints at the plant, and that different panels of the same car may come from different processes.
Limits
A gauge does not tell you why a panel was repainted, or how good the repair is. It is one tool alongside visual inspection (see good vs bad paint), panel gaps, and the condition of bolts and seals.