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.

Worked example

Nominal dry film thickness	300 um
Surface profile grade	medium
Number of readings taken	25
Mean gauge reading	340 um
Lowest gauge reading	270 um
Highest gauge reading	420 um
Readings between 80% and 100% of NDFT	3
Specified maximum DFT	900 um
Profile correction applied	25 um
Corrected mean DFT	315 um
Corrected minimum DFT	245 um
Corrected maximum DFT	395 um
80% minimum limit	240 um
Readings in the 80-100% band	12 %
Coating accepted	1

A medium profile takes 25 µm off every reading: mean 340 − 25 = 315 µm (above the 300 µm nominal), lowest 270 − 25 = 245 µm (above the 0.8 x 300 = 240 µm floor), highest 420 − 25 = 395 µm (below the 900 µm maximum). Three of 25 readings in the 80-100% band is 12%, under the 20% limit, so all four criteria are met.

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