Checking a car
How to spot a respray and filler with a thickness gauge
The seller says the car has never been hit or repainted, and it sounds convincing. Let’s see what the gauge actually shows, how to tell thick lacquer from a repair, and why the number on the screen is always compared rather than checked against a standard.
9 min read
The short answer: yes, it will
A thickness gauge measures the distance from its sensor to the metal — that is, the thickness of everything lying on top of the body. Factory coating stays within 80–150 µm, a resprayed panel gives 200–300, and a repair with filler starts at 500 µm and climbs. The difference between “never repainted” and the truth is about a hundred microns, and it shows on the screen in a second.
What separates thick lacquer from a repair is not the absolute figure but two things: agreement with the panel on the other side, and evenness within the panel itself. Lacquer goes on evenly; filler does not. And iron-bearing filler is flagged separately by a gauge with a detector, instead of being hidden inside the total thickness.
What the microns mean
Three reference points are enough to read the screen.
A cross-section: run the probe along the body
Gauge reading
120µm
Never repainted
Factory finish
Factory finish
120 µm
Repainted
200 µm
Body filler repair
620 µm
Drag across the section or use arrow keys
Layers are drawn to scale by thickness
What matters is not the absolute value but the evenness of the coating and the agreement between symmetrical panels. Factory thickness differs by make, generation and even production batch, so a reference table is a guide, not a standard.
The walk-around: compare, don’t check against a table
A single number means nothing. Comparison is what works: left wing against right, panel against neighbouring panel, and three points within each panel. Start with the roof — it is repainted least often and serves as the reference for the rest of this particular car.
Roof
Points: 118 · 121 · 115 µm
Spread within the panel: 6 µm
Factory coating. The figures are even and match the panel on the other side.
No pair: the roof and bonnet are judged against the whole car rather than a symmetrical panel.
factory coatingthickenedlikely repair
Every reading as a table
| Panel | Three points, µm | Verdict |
|---|---|---|
| Bonnet | 128 · 134 · 122 | normal |
| Left front wing | 124 · 130 · 119 | normal |
| Roof | 118 · 121 · 115 | normal |
| Right front wing | 268 · 291 · 255 | respray |
| Left front door | 127 · 133 · 121 | normal |
| Right front door | 240 · 615 · 680 | filler |
| Left rear door | 122 · 129 · 118 | normal |
| Right rear door | 134 · 141 · 126 | normal |
| Left rear wing | 130 · 125 · 137 | normal |
| Right rear wing | 143 · 150 · 138 | normal |
| Boot lid | 131 · 126 · 138 | normal |
In the example the right front wing gives an even 255–291 µm: the panel was resprayed as a whole, but there is no filler under the paint. The right front door reads 240 µm near the edge and 680 in the middle — that is the boundary of the repair, found by three points instead of one.
The spread between points is information, not a fault
Beginners are unsettled when two neighbouring points give different numbers. That is how it should be: coating on a body is never perfectly even, and it is the spread that reveals an uneven layer, the edge of filler and the boundary of a repair.
The opposite is what should worry you. Gauges with reading adjustment repeat the previous measurement when the new one lands close to it: the screen looks reassuringly steady while the body is less even than it appears. CARSYS gauges show every measurement anew.
Why an ordinary gauge does not see filler
Iron-bearing filler — the kind called magnetic — behaves under a magnetic field much like metal. A gauge without a detector takes it for the substrate and shows the thickness down to the filler rather than to the body: the screen gives an almost normal figure while a repair layer sits under the paint.
CARSYS body gauges flag such a repair separately: they do not blend it into the total thickness, they warn about it. On the DPM-816 line this is a triple warning signal.
You can test the detector without leaving home: the AVTOPODBOR kit includes a magnetic filler sample.
Detector threshold: adjustable only on the AVTOPODBOR line
The threshold is the point from which the gauge treats a find as filler. On the AVTOPODBOR line it can be moved to suit a particular body: lowered to catch a thin layer, or coarsened to show only clear findings. On the DPM-816 line the threshold is fixed — set with a margin against false positives.
Move the threshold and see what changes
Detector trigger threshold
70%
DPM-816 · fixed
Mid threshold — a balance of sensitivity and false-positive protection, close to the fixed DPM-816 setting.
Where the method does not work — honestly
On plastic bumpers, trim and fibreglass panels a gauge of this type does not measure at all: there is no metal under the coating to measure down to. The gauge will not be wrong — it simply gives no reading.
And second: the gauge shows coating thickness and signs of repair, not the history of the car. The conclusion about what happened is drawn by a person — from thickness, factory galvanising, panel gaps and paperwork.
Five mistakes that let a repair slip past
- 01Measuring through dirt and iceA layer of dirt, ice or water adds to the result. The body must be clean and dry, otherwise an inflated figure appears where there is no repair at all.
- 02One point per panelA single point will not catch the edge of filler: it may sit ten centimetres from where you measured. Three points per panel is a sensible minimum.
- 03Skipping the pillars and the roofThey are checked least often and repainted in any serious repair. Skip the pillars and a rebuilt car can easily pass for an intact one.
- 04Comparing panels that are not symmetricalA bonnet and a wing are painted differently even at the plant. Compare left with right and neighbour with neighbour, not anything with anything.
- 05Measuring on an edgeOn a rib or near the edge of a panel the measuring spot runs off the metal and the reading drifts. Step back from the edge.
Which gauge does this
Three things are needed for this kind of check. A resolution of 1 µm — otherwise the gauge rounds the result and the spread you were after disappears. Automatic recognition of steel and aluminium, so you are not switching modes at every panel. And a magnetic filler detector — without it a hidden repair stays invisible.
CARSYS body gauges have all three. The AVTOPODBOR line adds an adjustable detector threshold and a colour helper: green for factory coating, amber for thickening, red for a likely repair — the verdict is visible out of the corner of your eye, without reading numbers. The DPM-816 PRO offers five measurement modes and a comparison mode. Which suits you depends on how often and in what conditions you inspect cars.
Frequently asked
Does a thickness gauge see filler?
Yes, if the gauge has a magnetic filler detector. Without one, iron-bearing filler masquerades as metal and the gauge shows an understated thickness. CARSYS body gauges flag such a repair with a separate warning.
How do I know a panel was painted if the screen shows only 180 µm?
Compare it with the panel on the other side and with the roof. 180 µm on a car reading 120 everywhere is a thickening; on a car with 160 from the factory it is normal. The absolute figure decides nothing on its own.
Will the gauge show whether the car has been in an accident?
No. The gauge shows coating thickness and signs of repair. The conclusion about an accident is drawn by a person — from thickness, factory galvanising, panel gaps and paperwork.
Does a thickness gauge work on a bumper?
No. There is no metal under the plastic to measure down to, so bumpers, trim and fibreglass panels are not checked by this method.
How much can a difference of a few microns be trusted?
A spread of a few microns is normal: the AVTOPODBOR and DPM-816 PRO are accurate to ±1 % + 1 µm up to 500 µm, the DPM-816 LITE to ±2 % + 1 µm, and the coating itself goes on unevenly. What should worry you is not a five-micron difference but a difference of several times.
Knowledge base
Read next
Material that carries the topic on: how to check a body, what the gauge does and which one to choose.
Inspecting cars is easier with the right gauge
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