Buying task · Paint-thickness gauges
Paint thickness gauges: what each type can measure
Magnetic, eddy-current and combination paint gauges: which panels each can read, how makers state accuracy, and what to check before you buy one.
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Short answer
For a car, choose a combination Fe/NFe gauge: magnetic mode reads paint over steel and eddy-current mode reads paint over aluminum.[1] Neither mode reads plastic bumpers or fiberglass, which need an ultrasonic gauge. Then read the accuracy formula, not the display resolution, and plan to check the gauge on a known reference before you trust it.
What a paint gauge measures
A paint thickness gauge puts a probe on the panel and reports the thickness of everything between the probe and the metal underneath. DeFelsko says magnetic and eddy-current gauges measure the full distance to the base metal, including primer, base color, clear coat, any repaint, filler and dirt, with no distinction between layers.[2] So a reading is the whole coating thickness, not the clear coat alone.
Which method works depends on the substrate:
| Panel material | Method | Gauge mode |
|---|---|---|
| Steel | Magnetic | Fe |
| Aluminum and other non-ferrous metals | Eddy current | NFe |
| Plastic and fiberglass | Ultrasonic, with couplant gel | Not Fe or NFe |
DeFelsko gives this split for paint over steel, aluminum, plastic and fiberglass.[1] UNI-T adds that the object must be metal with a non-conductive coating for Fe/NFe readings.[3] The surface pages show each panel type in detail.
The decisions, in order
1. Which metals you need to read. A combination gauge covers steel and aluminum in one tool. DeFelsko's PosiTector 6000 probes, for example, come as F (ferrous), N (non-ferrous) and FN (both).[4] For a used car, where steel and aluminum panels can sit side by side, both modes matter.
2. Range and accuracy. Makers publish an accuracy band that grows with thickness. Examples from maker pages:
| Gauge | Range | Stated accuracy |
|---|---|---|
| DeFelsko PosiTest DFT (Ferrous or Combo) | up to 1,000 µm | ±3%[2] |
| Elcometer 311 (automotive) | 0-500 µm | ±5% or ±20 µm, whichever is greater[5] |
| Elcometer 456 integral (Scale 1) | 0-1500 µm | ±1-3% or ±2.5 µm, whichever is greater[6] |
| DeFelsko PosiTector 6000 (popular probes) | 0-1500 µm | ±(1 µm + 1%) up to 50 µm, ±(2 µm + 1%) above[4] |
| UNI-T UT343E | 0-2000 µm | ±(2%H + 2 µm)[7] |
| Linshang LS221 (split probe) | 0-5000 µm | ≤±(3%H + 2 µm)[8] |
These are each maker's own statements, not independent results. Elcometer's 311 figure also assumes a minimum substrate thickness, so read the footnotes.[5]
3. Accuracy is not resolution. Resolution is the smallest step the display shows; accuracy is how far the reading may be from the true value. UNI-T lists the UT343E's resolution as 0.1 µm below 100 µm but its accuracy as ±(2%H + 2 µm), so at 100 µm the reading can be about 4 µm out.[7]
4. How you will check it. ASTM D7091 describes calibration, verification and adjustment as three separate steps for accurate measurement.[9] A gauge that ships with zero plates or reference films makes the user check easier.
5. App and data features. Bluetooth logging is convenient, but it adds no accuracy. UNI-T, for example, lists app support and 3,300-reading storage for the UT343E.[7]
Every page in this topic
Guides
- Magnetic vs eddy-current paint gauges (Fe vs NFe)
- Paint gauge accuracy vs resolution: reading the spec sheet
- How to calibrate, verify and zero a paint gauge
- Can a paint gauge measure plastic bumpers or fiberglass?
- Can a paint gauge measure clear coat?
- What AUTO BILD's 2021 paint-gauge test shows, and its limits
- Bluetooth and app paint gauges: what the app adds
Tool
Surface pages
Using a gauge on a car you might buy is covered in the used-car paint inspection overview.
How to choose
- Pick a combination Fe/NFe gauge unless you only ever measure steel.
- Check the range covers the thickest reading you expect, with room above it.
- Read the accuracy formula and work it out at around 100 µm.
- Look for zero plates or reference films in the box, so you can check the gauge before each use.
- Check the seller before you pay: does the maker itself link the store?
UNI-T's where-to-buy page names a Singapore distributor and links no AliExpress or Amazon store.[10] Its UT343E page lists CE and UKCA as its certificates and no North American marks.[7]
A gauge shows thickness, not the cause. A high or uneven reading is an observation to follow up, not proof of a repaint or accident damage. The used-car paint check guide explains how to read a car panel by panel.
Next
Everything in this topic
Guides
- AUTO BILD's 2021 paint-gauge testHelps you use the one well-known independent gauge comparison without over-reading it.
- Bluetooth and app paint gaugesHelps you decide whether a Bluetooth gauge is worth it and how far to trust its app's verdicts.
- Can a paint gauge measure clear coat?Stops you using a total-thickness reading as a clear-coat reading before you polish.
- Can a paint gauge measure plastic bumpers?Explains why your gauge shows nothing on a bumper and whether an ultrasonic gauge is worth adding.
- How to calibrate, verify and zero a paint gaugeShows you how to check a paint gauge before trusting its readings, and when a lab calibration is worth paying for.
- Magnetic vs eddy-current paint gaugesHelps you choose between a steel-only gauge and a combination Fe/NFe gauge before you buy.
- Paint gauge accuracy vs resolutionStops you choosing a paint gauge by its display resolution instead of its stated accuracy and conditions.
Tools
- Paint gauge surface finderMatches each panel on a car to the gauge method that can read it, so you know before you measure whether a blank or odd reading is expected.
Common questions
Is the AUTO BILD paint gauge test recent?
No. The page dates its prices to 20 May 2021, so it is a 2021 test of gauges sold in Germany at the time, even though the page is still online. More in AUTO BILD's 2021 paint-gauge test
Does the test show NexPTG gauges are inaccurate?
It shows that one NexPTG Professional sample deviated by up to 118% from AUTO BILD's reference device in 2021. That is one unit of one model, not every NexPTG gauge or current models. More in AUTO BILD's 2021 paint-gauge test
Are cheap paint gauges unreliable?
AUTO BILD found implausible values from three specific budget models, including one reading above 2000 µm on a gauge rated to 1000 µm. That is a warning to verify any gauge, not a rule for all budget gauges. More in AUTO BILD's 2021 paint-gauge test
What did AUTO BILD use as its reference?
The Q-Nix 5500, because it was the most precise of the group on plastic control foils from 48 to 1006 µm. It was not an accredited laboratory master. More in AUTO BILD's 2021 paint-gauge test
Does AUTO BILD say 200 µm means repainted?
It gives factory finishes as usually 80-150 µm and manual repaints as typically above 200 µm, but calls these only rough guidance and notes that factories sometimes repaint. It is not a threshold. More in AUTO BILD's 2021 paint-gauge test
Does a Bluetooth paint gauge measure more accurately?
No. The app shows what the probe measured, and accuracy stays what the maker states for the gauge, for example ±(2%H + 2 µm) for UNI-T's UT343E. More in Bluetooth and app paint gauges
Can the app tell me a panel was repainted?
Some apps color-code panels; NexDiag's NexPTG app labels them factory, repaired or repainted with its SAPL system. That is the maker's interpretation of a thickness number, not proof of a repair. More in Bluetooth and app paint gauges
Does every NexPTG gauge make PDF reports?
No. NexDiag says PDF reports are not supported on its NexPTG Standard and Standard Plus gauges. More in Bluetooth and app paint gauges
Questions
Can one paint gauge measure every panel on a car?
Not with magnetic or eddy-current probes alone. DeFelsko says paint over steel is measured with magnetic gauges, paint over aluminum with eddy-current gauges, and paint over plastic and fiberglass with ultrasonic gauges.[1]
What do Fe and NFe mean on a paint gauge?
UNI-T explains that FE means magnetic substrates such as iron and steel, and NFE means non-magnetic metals such as aluminum and copper. A combination (Fe/NFe) gauge covers both.[3]
Is a gauge that shows 0.1 µm accurate to 0.1 µm?
No. Resolution is the display step, accuracy is the error band. UNI-T lists the UT343E's resolution as 0.1 µm below 100 µm but its accuracy as ±(2%H + 2 µm).[7]
Do I need a professional gauge to check a used car?
You need one that reads both steel and aluminum and that you check before use. Automotive meters such as DeFelsko's PosiTest DFT Combo and the Elcometer 311 FNF are made for this, and the makers publish their ranges and accuracy.[2][5]
Is there a standard for paint gauges?
ASTM D7091 covers magnetic and eddy-current gauges for dry film thickness and describes calibration, verification and adjustment as three steps for accurate measurement. It is an industrial practice that supplements the gauge maker's instructions.[9]