Dry film thickness acceptance

Magnetic and eddy-current dry film thickness gauges are calibrated on smooth steel, but protective coatings are applied over a blast profile. The probe sits on the peaks while the gauge averages a field that reaches into the valleys, so on a rough surface the instrument reads high — by an amount that depends on how coarse the profile is. ISO 19840 handles this with fixed correction values subtracted from every reading: 10 µm for a fine profile, 25 µm for medium and 40 µm for coarse, using the ISO 8503 comparator grades. On a 300 µm specification over a medium profile that correction is over 8% of the film, which is easily the difference between acceptance and rejection.

Acceptance itself is the 80/20 rule, and all four of its parts have to be satisfied. The arithmetic mean of the corrected readings must be at least the nominal dry film thickness. No individual reading may fall below 80% of the nominal. The number of readings that fall between 80% and 100% of nominal must be fewer than 20% of the total. And no reading may exceed the specified maximum thickness — over-application is a genuine defect, causing solvent entrapment, cracking and disbonding, not a free margin of safety.

The corollary is that a healthy mean does not rescue a coating with thin spots. A single reading below the 80% limit fails the area regardless of how generous the average is, which is why gauge readings should be taken across the whole area with attention to edges, welds, bolt heads and other geometry where film thins, rather than clustered on flat plate where the spray was easy.

Two conditions have to hold before any of this is meaningful. The gauge must be verified on certified shims over the actual blasted substrate, per ISO 2808 and the gauge manufacturer's instructions, at the start and end of each session. And the correction values apply to blast-cleaned steel with a profile within the comparator grades — on hand-cleaned, machined or previously coated surfaces the correction has to be established by measuring the profile directly and agreeing a value with the specifier.

corrected DFT = gauge reading − profile correction (fine 10, medium 25, coarse 40 µm, or the measured/agreed value)
mean corrected DFT ≥ NDFT
every individual DFT ≥ 0.8 × NDFT
count of readings between 0.8·NDFT and NDFT < 20% of total
no individual DFT above the specified maximum