Guide · Used-car paint inspection
Can a paint gauge detect body filler?
A paint gauge reads filler as extra thickness, or shows nothing past its range. How filler shows up, the steel-particle and tin catches, and what it cannot prove.
Short answer
Often, indirectly. A magnetic or eddy-current gauge reads everything between the probe and the metal, filler included,[1] so a filled area can read much thicker than the paint around it, or give no reading at all when the total exceeds the gauge's range.[2] But it cannot tell filler from thick paint, some magnetic gauges can be fooled by filler containing steel particles,[2] and tin-filled repairs on steel need a gauge with NFe capability.[3]
How filler shows up
DeFelsko says magnetic and eddy-current gauges measure the full distance from the probe to the base metal, including primer, color, clear coat, any repaint, filler and dirt, with no distinction between layers.[1] Body filler adds to that distance, so it raises the reading.
In a DeFelsko case study, readings around a vehicle were 105-115 µm. One lower front panel read 270 µm and then 930 µm, which turned out to be plastic filler over a rust repair.[2] That is one vehicle, measured with a PosiTest DFT Combo. It shows the pattern, a local jump far above the rest of the car, not a number to apply to other cars.
When the gauge shows nothing
DeFelsko says that if the gauge gives no reading, the thickness may exceed its limit, which could mean filler.[2] Before you conclude that, rule out the simpler causes:
- Wrong mode. An Fe-only gauge on an aluminum panel, or NFe mode on steel.
- Plastic part. Fe/NFe gauges cannot read plastic or fiberglass.[2]
- Edge or curve. Move to a flatter spot about one inch from the edge.[2]
If a metal panel reads normally all around one spot and gives nothing in the middle, that spot deserves a closer look.
Two catches
Steel particles in filler. DeFelsko warns that, even on steel, some magnetic instruments can be fooled when steel particles are added to body filler.[2] A filled area could then read closer to normal than it should. This is a statement about some instruments, not a measured failure rate.
Tin filler. Tin filler is a metal filler rather than a plastic one. AUTO BILD notes that tin-filled repairs on a steel body can only be found with a gauge that also has NFe (eddy-current) capability.[3] A steel-only gauge can miss them. See tin-filled repairs.
What a filler reading does and does not tell you
| The gauge can show | The gauge cannot show |
|---|---|
| Where the coating is much thicker than around it | Whether it is filler, thick paint or something else |
| Roughly how large the thick area is, if you measure a grid | How well the repair was done |
| That a spot may be beyond the gauge's range | What is under the filler |
A thickness number, on its own, is not proof of filler or of accident damage. If a panel shows a local jump, ask for the repair history and have a body shop look at it before you buy.
Next
Questions
What does body filler read like on a paint gauge?
As much thicker coating than the paint around it. In one DeFelsko case study a panel read 270 µm and then 930 µm against 105-115 µm elsewhere, and it turned out to be plastic filler over a rust repair.[2]
Why does my gauge show no reading on one spot of a metal panel?
DeFelsko says no reading may mean the thickness exceeds the gauge's limit, which could mean filler. Check the mode, the edge distance and that the part is metal first.[2]
Can filler fool a paint gauge?
DeFelsko warns that some magnetic instruments can be fooled when steel particles are added to body filler, even on steel.[2]
Can a steel-only gauge find tin filler?
No, according to AUTO BILD: tin-filled repairs on a steel body can only be found with a gauge that also has NFe (eddy-current) capability.[3]