What a paint thickness gauge reveals

When we inspect a car before purchase, one of the tools we use most is the paint thickness gauge. It is a small device you rest on the bodywork, and in an instant it shows the thickness of the layers sitting on top of the metal. It may not look like much, but it gives very valuable information about the car's history.

How it works

Factory paint has several layers: the protection on the metal, the primer, the colour and the clear coat. Together they add up to a very thin thickness, measured in microns (thousandths of a millimetre), and fairly consistent across the whole car. The gauge detects the distance between its tip and the metal underneath and shows it on screen. Depending on the type of device, it works on steel, aluminium or both.

What it can reveal

• Repainted panels. When a panel is repainted, layers are added over the originals or replace them. The result is usually a thickness that differs from the rest of the car.
• Areas with filler. Body filler is used to level out irregularities after a knock. Where there is filler, the gauge shows a much higher reading, sometimes so high it goes off the scale.
• Replaced panels. A new replacement panel may have a different thickness from the original, sometimes lower, which is also a clue.
• Localised repairs. Measuring at several points on the same panel shows whether the work covers the whole surface or just one area.

How to read it

A single reading says little. What matters is comparison: between one panel and the next, between left and right sides, and within the same panel. So in an inspection we systematically measure every panel, at several points each, and record the results. If all panels give similar readings, that is a good sign. If one door or wing stands out, it deserves a closer look.

What's more, each manufacturer and model has its own factory values, and some cars leave the factory with small differences between panels. That is why interpreting the readings takes experience and context.

What it cannot tell you

The gauge is very useful, but it has limits worth knowing:

• It does not measure on plastics with the usual devices. Bumpers, mirrors and other plastic parts have to be checked in other ways.
• It does not say why a panel was repainted. It could be a heavy knock or just a scratch. A respray is not necessarily a bad thing.
• It does not assess the structure. A car can have all its original paint on the outside and still have damage to rails or inner reinforcements, or the other way round.
• It does not replace a trained eye. It has to be combined with a visual inspection, with the car raised, checking joints, welds and panel fit.

Why it matters when buying

Knowing that a panel has been repainted lets you ask the seller informed questions, judge whether the price is fair and, if something does not add up, dig deeper before paying. It is useful for sellers too: a report showing which panels keep their original paint gives buyers confidence.

In our pre-purchase inspection we measure every metal panel and record it in a bodywork and paint report with photos and explanations. If you are selling, you can also ask for a pre-sale inspection.