Inspection method · DFT
Coating thickness and coating inspection
Coating protection is measured in microns. The right gauges, sampling and acceptance rules decide whether a coating meets its specification.
01How it works
Dry film thickness (DFT) on steel is measured with magnetic induction gauges; on non-ferrous metals with eddy current gauges. Gauges are verified with calibration foils on a substrate similar to the work. Wet film thickness (WFT) is measured with comb gauges during application to control the final DFT.
Coating inspection also includes surface preparation grade (visual comparison with ISO 8501-1), surface profile, dust and soluble salts, climatic conditions (air and steel temperature, relative humidity, dew point) and adhesion tests such as pull-off or cross-cut.
02Typical industrial applications
- Painted structural steel
- Tanks and pressure equipment
- Machinery and equipment
- Hot-dip galvanized products
- Powder coated parts
- Marine and offshore coatings
03Benefits and what it detects
- Quick and quantitative
- Verifies specification compliance
- Prevents premature corrosion
- Documents conditions during application
04Limitations
- Gauge calibration and substrate effects
- Sampling rules affect acceptance
- Adhesion tests can be destructive
- Surface preparation cannot be checked after coating
05Specifying and reviewing the method
Specify coating system, DFT per coat and total, sampling and acceptance rules, surface preparation grade and climatic limits. Use the DFT calculator to relate WFT to DFT.
Other references: ISO 19840, SSPC-PA 2, ISO 4624 (pull-off), ISO 2409 (cross-cut)