Guide · Used-car paint inspection
How to check a used car's paint with a gauge
A step-by-step paint gauge check for a used car: verify the gauge, identify each panel's material, measure consistently, and compare without over-reading.
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Short answer
Check the gauge first, find out what each panel is made of, then measure every panel the same way and compare. DeFelsko advises looking for consistent thickness around the car and measuring near seams about one inch from the edges.[1] AUTO BILD adds a mirrored-spot comparison, left side against right.[2] The readings tell you where the paint is thicker or thinner; they do not tell you why.
What you need
- A combination Fe/NFe gauge, so steel and aluminum panels both read. AUTO BILD notes that tin-filled repairs on a steel body can only be found with a gauge that also has NFe capability.[2]
- The zero plate and shims that came with the gauge.
- A panel map: a simple sketch of the car to write readings on, or the gauge's app.
- A clean microfiber cloth. Dirt under the probe counts as coating.[3]
Step 1: verify the gauge
Before you start, measure an uncoated part (a zero check), then the included shims laid on bare metal. DeFelsko says cheaper instruments drift and should be checked before use each day.[3] If the shim readings are off, sort that out first: see how to calibrate and verify a paint gauge.
Step 2: find out what each panel is made of
DeFelsko notes that recent cars can combine steel doors and fenders, aluminum hoods and roofs, and plastic bumpers and mirrors.[1] Steel reads in Fe mode, aluminum in NFe mode, and plastic or fiberglass not at all with this kind of gauge.[1]
A combination gauge that identifies the substrate automatically shows which mode it used. A small magnet also helps: it sticks to steel, not to aluminum or plastic. Mark plastic parts on your map as "not measured" rather than leaving them blank.
Step 3: measure the same way everywhere
- Hold the probe flat and steady on a clean, flat area.
- Stay about one inch from edges and seams, as DeFelsko advises.[1]
- Take several readings per panel: front, middle and rear of a door; corners and center of a hood or roof.
- Repeat a reading that looks odd before you write it down. One reading is not a finding.
- Write down every number on the map, with the mode (Fe or NFe).
Step 4: compare, do not judge single numbers
- Around the car. DeFelsko advises looking for consistent thickness around the vehicle.[1]
- Adjacent panels. Compare a door with the fender next to it.
- Mirrored spots. AUTO BILD advises comparing the same spot on each side: a much higher reading on one side indicates a repair with repaint, in its view.[2]
There is no single "normal" number to compare against. Published typical ranges differ between sources: see normal car paint thickness.
Step 5: read the results carefully
| What you see | What it can mean | What to do |
|---|---|---|
| Consistent readings around the car | Nothing stands out on the metal panels | Still look at plastic parts and the paint in good light |
| One panel clearly thicker than its neighbors | Extra paint or other material on that panel | Re-measure, look closely, ask the seller about its history |
| One side higher than the mirrored spot | AUTO BILD reads this as a repair with repaint[2] | Treat as a question, not a verdict |
| No reading on a metal panel | DeFelsko says thickness may exceed the gauge's limit, which could mean filler[1] | Check the mode and the gauge's range; see body filler |
| No reading on a bumper | Plastic, as expected | Note it as "not measured" |
DeFelsko goes further than we do: it says a significant difference between adjacent panels strongly indicates repainting.[1] Our view is that the gauge shows thickness, not the cause. Factories sometimes repaint too.[2] A difference is a reason to look closer and ask, not proof of an accident.
Units on the screen
Most gauges switch between microns and mils. One mil is 25.4 µm.[3] Pick one unit for the whole car.
When to bring in a professional
If you find panels that stand out, a gauge cannot tell you whether the repair was good. For a purchase that matters, ask a body shop or an independent pre-purchase inspector to look at those panels.
Next
Questions
Where should I measure on each panel?
Several points per panel on clean, flat areas, staying about one inch from edges and seams as DeFelsko advises, and the same spots on both sides of the car.[1]
Does a thicker panel mean the car was in an accident?
Not by itself. A gauge shows thickness, not the cause, and AUTO BILD notes factories sometimes repaint. Treat a difference as a reason to look closer and ask.[2]
Why do I get no reading on part of the car?
On a bumper or mirror the part may be plastic, which Fe/NFe gauges cannot read. On metal, DeFelsko says no reading may mean the coating is beyond the gauge's limit, which could mean filler.[1]
Do I need a gauge with NFe mode for a steel car?
It helps. AUTO BILD notes tin-filled repairs on a steel body can only be found with a gauge that also has NFe capability, and many cars mix steel and aluminum panels.[2][1]
Should I check the gauge before every car?
Yes. DeFelsko says cheaper instruments drift and should be checked before use each day: a zero check on bare metal, then the included shims.[3]