📏 Paint coating thickness
Non-destructive measurement of paint and galvanic coating thickness by magnetic and eddy current methods. Quality control in serial production.
Overview
Coating thickness measurement is one of the most frequently performed tests in paint and galvanic production quality control. The PN-EN ISO 2808 standard describes many measurement methods, of which two non-destructive ones — magnetic method (method no. 6) and eddy current method (method no. 7) — are most widely used in industry.
The magnetic method (according to ISO 2178) is used to measure thickness of non-magnetic coatings (paint, zinc, chromium, copper) on ferromagnetic substrates (steel, cast iron). It uses the principle of magnetic induction — change in probe distance from ferromagnetic substrate (caused by coating presence) changes electromagnetic signal proportionally to layer thickness.
The eddy current method (according to ISO 2360) is used to measure thickness of non-conductive coatings (paint, anodizing, powder paint) on non-magnetic metal substrates (aluminum, copper, brass). A coil in the probe generates an alternating magnetic field at frequency >1 MHz, which induces eddy currents in the substrate — their intensity depends on probe distance from metal, and thus on coating thickness.
In the automotive industry, coating thickness measurement is mandatory at every stage of the painting process: KTL (15–25 µm), primer (30–40 µm), base (12–18 µm), clear coat (35–50 µm). Exceeding or underestimating thickness can result in aesthetic defects, reduced corrosion resistance, or interlayer adhesion problems.
Method principle
Magnetic method: The measuring probe contains a permanent magnet or electromagnet. When approaching a ferromagnetic substrate (steel), a signal proportional to magnetic interaction force is generated. Non-magnetic coating (paint, zinc) increases probe distance from substrate, weakening signal — this change is converted to coating thickness after calibration on standards. Eddy current method: A coil powered by alternating current at high frequency (>1 MHz) generates an alternating magnetic field. Eddy currents are induced in non-magnetic metal substrate (aluminum), whose impedance depends on coil distance from metal. Non-conductive coating (paint, anodizing) increases this distance, changing impedance — coating thickness is calculated based on this.
Applications
- Coating thickness control in automotive painting process (KTL, primer, base, clear)
- Measurement of galvanic coating thickness (zinc, nickel, chromium) on fasteners
- Anodizing thickness control of aluminum in aviation and architecture
- Powder paint thickness measurement on metal furniture and appliance components
- Incoming inspection of components from sub-suppliers
- Testing anticorrosive coating thickness on steel structures
- Verification of thickness after paint repairs (bodywork, detailing)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range (magnetic) | 0–3 000 µm (typically 0–1 500 µm) |
| Measurement range (eddy current) | 0–2 000 µm (typically 0–1 000 µm) |
| Accuracy | ±(1 µm + 1%) or ±(1 µm + 3%) depending on instrument |
| Resolution | 0.1 µm (laboratory instruments), 1 µm (industrial) |
| Minimum measurement area | Ø 5–10 mm (depends on probe diameter) |
| Calibration | On substrate identical to tested, standard calibration foils (e.g., 25, 50, 125, 250 µm) |
Standard
- Standard number
- PN-EN ISO 2808:2020
- Title (PL)
- Farby i lakiery — Oznaczanie grubości powłoki
- Title (EN)
- Paints and varnishes — Determination of film thickness
Step-by-step procedure
1. Coating system identification
Determine substrate type (ferromagnetic → magnetic method, non-magnetic → eddy current) and coating type. Select appropriate probe.
2. Zero calibration
Apply probe to clean substrate without coating (same material and shape as tested element). Set instrument zero.
3. Calibration on standard foils
Apply certified calibration foils to reference substrate. Perform measurements and calibrate instrument (1–2 calibration points).
4. Calibration verification
Measure foil of known thickness — result should be within tolerance ±(1 µm + 1–3%). If not — repeat calibration.
5. Coating thickness measurement
Apply probe perpendicular to surface. Perform minimum 5 measurements at different points (according to measurement pattern or randomly). Read results.
6. Curvature and edge effect assessment
On curved elements or near edges, use small-diameter probes. Avoid measurements <10 mm from edges.
7. Statistical analysis
Calculate average, standard deviation, minimum and maximum value from measurement series. Compare with specification requirements (min/max/average).
8. Documentation
Record: instrument type, probe, calibration date, measurement point, individual measurement results, statistics, compliance assessment.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Coating thickness gauge (dual FN) | DeFelsko PosiTector 6000, Elcometer 456, Fischer FMP30/FMP40 | 2 000–15 000 PLN |
| Interchangeable probes | Fe probes (magnetic), NFe (eddy current), FNF (dual), micro, angle | 1 000–5 000 PLN/pc |
| Certified calibration foils | DeFelsko, Elcometer, Fischer — set of 5-8 thicknesses with certificate | 300–800 PLN/set |
| Calibration substrate plates | Steel / aluminum, ground, with certificate | 200–600 PLN/pc |
| Metallographic microscope (opt.) | Olympus BX53M, Zeiss Axio Observer — cross-section measurement (destructive method) | 80 000–250 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Calibration foils | — | Set of certified foils with known thicknesses (25, 50, 125, 250, 500 µm), traceable to national standard |
| Metallographic embedding resin | — | Epoxy or acrylic (e.g., Struers EpoFix) — for cross-section preparation (destructive method) |
| Abrasive papers and polishing pastes | — | SiC P320–P4000 + diamond paste 3–1 µm — for grinding and polishing cross-sections |
Health and safety (OHS)
- Measuring probes — delicate, protect from dropping and impact
- Calibration foils — do not bend, store flat in packaging
- When measuring on sharp elements — protective gloves
- Electronic instruments — protect from moisture and solvents