Dry film thickness (DFT) is the most common measurement in coating inspection, and one of the most misunderstood. ISO 19840 sets out how to measure DFT on rough, blast-cleaned steel and how to decide whether the result is acceptable.
The problem with rough steel
A magnetic or eddy current gauge measures from a point somewhere within the blast profile, not from the top of the peaks. So a gauge calibrated on smooth steel over-reads on a blasted surface. ISO 19840 deals with this through correction values that depend on the profile grade (fine, medium or coarse, see ISO 8503), unless the gauge is adjusted on a representative blasted surface.
Sampling
The standard defines how many readings to take for the size of the area being inspected, so larger areas get more readings, and it tells you how to group them. The specification may set its own sampling plan instead.
The 80:20 acceptance rule
Unless the specification says otherwise, ISO 19840 accepts a coated area when:
- The average of all readings is equal to or greater than the nominal dry film thickness (NDFT).
- Every reading is at least 80% of the NDFT.
- Readings between 80% and 100% of the NDFT make up no more than 20% of the total.
- No reading exceeds the maximum thickness set in the specification or data sheet.
Too much paint is a defect too: excessive thickness can cause cracking, poor cure and trapped solvent.
ISO 19840 or ISO 2808?
ISO 2808 describes the methods for determining film thickness in general. ISO 19840 applies specifically to rough surfaces and adds the correction and acceptance rules. The specification should say which applies.
How we help
We measure DFT with calibrated gauges, report the readings and statistics against the specification, and explain any non-conformance clearly. See our inspection service.
This guide summarises the standard for general understanding. Always work to the current edition and the project specification.