Automotive paint thickness
Read microns and mils correctly, match the gauge to the substrate and separate total thickness from clearcoat allowance.
Know what the instrument measures
Paint thickness is the thickness of coating above a substrate. Before interpreting the number, establish whether the instrument reports the whole coating or can distinguish an individual layer. A reading is not automatically a measure of remaining clearcoat.
One micrometre, written μm or called a micron, is 0.001 mm. Thus 120 μm is 0.12 mm. One mil is 25.4 μm; a mil is not a millimetre.
The total can contain several layers
Primer, colour, clearcoat and repair materials can contribute to a reading, depending on the measurement method. On steel, other non-magnetic coatings can also be included. A total of 120 μm does not mean 120 μm of clearcoat.
Do not calculate clearcoat allowance by assigning a fixed percentage of the total. Layer measurement is available in certain suitable instrument and coating combinations, but an ultrasonic label alone does not guarantee that capability on every car.
Match the method to the substrate
Magnetic methods are used for suitable non-magnetic coatings on ferrous substrates. Eddy-current methods can measure suitable non-conductive coatings on non-ferrous metal such as aluminium. Non-metal panels require a method designed for that substrate and coating, such as a suitable ultrasonic gauge.
Some gauges support both ferrous and non-ferrous metals, but not every gauge switches automatically. No reading does not mean there is no paint: check substrate, range and probe contact first.
Measure a map, not one convenient point
Check the unit, mode and instrument verification or adjustment procedure. Use the prescribed standards; do not strip a car’s paint to obtain a bare calibration area. Clean and dry the surface and place the probe consistently. Curvature, edges and roughness can affect results.
Measure several locations and repeat unusual readings. Record the panel, location, substrate, mode and values. Keeping only an average loses useful local differences. Revisit the same points when comparing before and after work.
Use readings as part of the correction decision
A high value can justify further investigation of repair history, but it does not prove a particular repair or establish safe sanding. Combine readings with paint condition, cracks, lifting, defect depth and uncertainty about previous removal.
No universal minimum thickness, amount removed per polish or lifetime polishing count guarantees safety. A small difference may fall within instrument accuracy or repeatability. Unchanged displayed numbers do not prove that abrasion removed nothing.