Quick answer: For a nonmagnetic powder coating on steel, use a magnetic-induction coating thickness gauge. For a nonconductive powder coating on aluminum, use an eddy-current gauge. Confirm the metal and coating combination first, verify the gauge on a representative uncoated part, and measure after cure. The contract or powder supplier’s specification—not a universal number—sets the target dry-film thickness.
Choose the method before choosing a reading
| Part beneath the powder | Usual non-destructive method | What to check |
|---|---|---|
| Magnetic steel | Magnetic induction (Fe) | The powder is nonmagnetic; confirm the part is ferromagnetic. |
| Aluminum or another suitable nonmagnetic conductive metal | Eddy current (NFe) | The powder is nonconductive; confirm the substrate is conductive. |
| Plastic, wood or concrete | Neither ordinary Fe nor NFe mode | Use a method validated for the actual substrate and coating. |
ISO 2178 covers nonmagnetizable coatings on magnetizable base metals; ISO 2360 covers nonconductive coatings on nonmagnetic conductive base metals. Both explicitly include powder coatings within their scope. A dual-mode gauge can switch methods, but automatic detection should still be checked against the known substrate. See magnetic induction and eddy current for the underlying principles.
A six-step field procedure
- Identify the part. Record substrate, coating system, part geometry and the specification that defines acceptance.
- Wait for the specified cure. A dry-film probe pressing into a soft or uncured film can change the result; follow the powder supplier’s cure instructions.
- Check the instrument. Verify its calibration status and select a range and probe suited to the expected thickness and part shape.
- Adjust and verify on a representative bare part. Follow the gauge instructions; use a known shim or reference thickness on an uncoated area of comparable metal, roughness and geometry.
- Measure a planned set of locations. Include representative faces and known risk areas; avoid treating a single edge reading as the whole part. Record each reading and location.
- Judge against the actual specification. Separate the instrument’s readings from the project’s sampling and acceptance rule. ASTM D7091 describes gauge operation and verification; it does not supply one powder thickness target for every job.
Frequent mistakes
Fe mode on aluminum, NFe mode on steel, verification on a smooth plate for a rough production part, and an undocumented change of probe or zero setting can make results incomparable. If a part has zinc under the powder, treat it as a multilayer system rather than a simple bare-steel measurement; see our duplex-coating guide.
What belongs in the report?
Keep the part and zone ID, substrate, coating and cure state, gauge and probe IDs, calibration and verification records, individual readings, measurement locations and the cited acceptance document. This field-record template shows how to make a conclusion auditable.
Questions buyers ask
Can one Fe/NFe gauge work on both metals?
Yes, if its specified modes and range cover the two jobs. Verify each mode on a representative substrate before using it.
What thickness should powder coating be?
There is no single number for every powder, part or service environment. Use the approved coating specification and the powder manufacturer’s data sheet.