Paint thickness readings, and what they actually tell you

A paint thickness gauge is the most useful cheap instrument in a pre-purchase examination, and the most commonly misread. It gives you a number in microns, buyers treat that number as a verdict, and the number on its own means very little. What the tool is genuinely good at is comparison: telling you that this panel is not like the panels around it.

What the number is

A micron is a thousandth of a millimetre. The gauge measures the total distance from the surface down to the metal, so it is adding up primer, colour coat and clearcoat, and anything else in between such as filler. It cannot distinguish those layers. A reading of 200 microns does not tell you whether that is thick paint or thin paint over filler, and that difference is the one you care about.

This is why the tool is a pointer rather than an answer. It tells you where to look with your eyes and hands.

Typical ranges, with the caveat that matters

As orientation, most volume-production cars leave the factory in the region of 80 to 150 microns. Some premium makers run thicker. Certain colours and finishes differ. Panels on the same car can vary by design, and cars of the same model built at different plants can differ. So published ranges are worth knowing and are not a standard to hold a car against.

What is much more reliable is internal consistency. A car that reads in a tight band across every steel panel has almost certainly never been repainted. A car that reads in a tight band except for one wing has been repaired at that wing, whatever the absolute figures are.

Build a panel map, not a reading

Take several readings per panel, not one, because paint varies across a surface. Cover the bonnet, roof, both front wings, all four doors, both rear quarters, and the boot lid or tailgate. Write them down as you go. Patterns then read themselves:

• One panel high, rest even. A localised repair. Usually unremarkable, and worth confirming against the story you were told.
• Several adjacent panels high. This is the one to take seriously. Damage that crossed panel boundaries was not a parking scrape.
• One side of the car consistently higher. A side impact repair, or a car that was resprayed along one flank.
• The roof reading high. Roofs are rarely repainted for cosmetic reasons. Hail is the benign explanation. The other explanations are not benign.
• Wildly high, far above neighbours. Filler. The question is no longer about paint.

Read it together with panel alignment

The gauge is much stronger when combined with your eyes. Check panel gaps for evenness along their length, look down the side of the car in raking light for ripples, open and close every door and the boot to feel for anything that binds, and compare the texture of the paint surface between panels. Factory finishes have a consistent orange-peel texture; a resprayed panel often differs subtly, and once you have noticed it on one car you will see it everywhere.

A high reading plus a wide gap plus a slightly different texture is a much stronger conclusion than any of the three alone.

Bumpers are a separate story

Most gauges work by magnetic induction and need steel underneath. Bumpers are usually plastic, so the tool either reports nothing or reports nonsense. This confuses people into recording a bumper figure as a finding. It is not one. Bumpers also get repainted constantly for entirely trivial reasons, so even a confirmed bumper respray tells you little.

What to hold on to when deciding

Repaint is not damage. Damage is damage, and paint is one of several clues that damage happened. A car with a repainted door and a documented reason is often a better buy than an untouched car with no service records at all. What you are looking for is a story that holds together: readings that match what the seller said, a repair history that explains the panels that read high, and no structural implication behind any of it.

Where the readings suggest something crossed panel boundaries, that is the moment to move on to structural checks, because paint over filler over reshaped metal is a different purchase from a resprayed wing. The full pre-purchase sequence puts this step in order with the rest.