Pro edition
Paint Depth Log.
A gauge reading on its own tells you nothing. It becomes useful when you know the band it falls in, what the panel beside it reads, and how many microns the pass you are about to make will spend. All three are on this sheet.
Reading bands
| Reading | What it means | What to do |
|---|---|---|
| Under 25 µm | At risk. Do not cut. | Protect only. Coating or sealant, no machine correction. |
| 25–35 µm | Thin. Previously corrected, or a light factory coat. | One finishing pass at most, and only if the defect warrants it. |
| 36–55 µm | Normal factory range. | Full correction options available. |
| 56–90 µm | Heavy factory coat, or a blend edge. | Treat as factory but re-measure across the panel. |
| Over 100 µm | Refinished. Almost certainly repainted. | Lighter pass — refinish clear is softer and cuts faster. |
A factory panel at the bottom of the normal range (40 µm) has about 15 µm of usable clear above the 25 µm floor. Everything below is how that gets spent.
What each pass spends
| Pass | Typical spend | Range |
|---|---|---|
| Wash and decontaminate | 0 µm | Nothing is removed from the paint. |
| Finishing polish | 2 µm | Typical. Range 1–3. |
| One-step correction | 4 µm | Typical. Range 3–5. |
| Two-step correction | 8 µm | Typical. Range 6–10. |
| Wet sanding, spot | 10 µm | Typical. Range 5–15, measured panel only. |
Taking the readings
- Take three readings per panel: top, middle and bottom. Paint is not uniform.
- Compare against the neighbouring panel, never against a table. The step between panels is what identifies a repaint.
- Plastic bumpers read differently from steel. Do not compare them to metal panels.
- Aluminium panels read differently again on some gauges. Know which your gauge compensates for.
- Record the reading before any machine work, not after. Afterwards it is evidence of what you took.
Blank sheet
Three readings per panel. Print it, fill it in on the car.
| Panel | Top | Middle | Bottom | Verdict |
|---|---|---|---|---|
| Bonnet | ||||
| Roof | ||||
| Boot lid | ||||
| Front wing, left | ||||
| Front wing, right | ||||
| Front door, left | ||||
| Front door, right | ||||
| Rear door, left | ||||
| Rear door, right | ||||
| Rear quarter, left | ||||
| Rear quarter, right | ||||
| Front bumper | ||||
| Rear bumper |
Before you agree to anything
- Ask what the panel measured, not what the car measured. There is no such thing as a car reading.
- Ask how many microns the proposed pass is expected to remove.
- Ask what the reading will be afterwards, and whether that leaves room for a next time.
- If a shop does not own a gauge, it is estimating with something it cannot replace.
- Get the survey in writing. It is the baseline for anything that happens later.
Typical figures. Clear coat thickness varies by manufacturer, model year and panel, and gauges differ in how they compensate for substrate. Measure the car in front of you.