Wet Film Gauge Says Pass, Dry Film Says Fail
An applicator is spraying a high-build epoxy at a specified dry film thickness (DFT) of 250–350 µm over a blasted hull. During application the inspector checks wet film thickness (WFT) with a comb gauge and records readings of 300–340 µm across the strake. The applicator's target WFT is calculated using a volume solids of 65%. After cure, magnetic DFT readings on the same strake average 210 µm, with several readings at 190 µm. The owner's specification requires 80% of readings at or above the minimum DFT and no reading below 80% of the specified minimum. The applicator insists the WFT readings prove the DFT is in specification.
1. Reconstruct the WFT-to-DFT relationship and explain why the wet readings and dry readings disagree.
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The relationship is: DFT = WFT × volume solids (as a fraction). With 65% volume solids, a 300–340 µm wet film should yield roughly 195–221 µm dry - which matches the measured 190–210 µm DFT almost exactly. The wet readings and the dry readings do not disagree; they are consistent. The applicator's error is in the target, not the measurement.
To hit 250–350 µm DFT at 65% volume solids, the required WFT is:
- For 250 µm DFT: 250 / 0.65 ≈ 385 µm wet
- For 350 µm DFT: 350 / 0.65 ≈ 538 µm wet
So the recorded 300–340 µm wet film was always going to produce roughly 195–220 µm dry - below the 250 µm minimum. The applicator was spraying to a WFT target appropriate for a higher-solids product, or simply miscalculated. Either way, the wet gauge was telling the truth; the target was wrong.
This is a classic volume-solids trap. The inspector should always verify the WFT target against the actual product data sheet volume solids, not assume the wet number looks 'high enough.'
2. Apply the owner's acceptance criteria to the DFT data and state the disposition.
Read the worked answer
Owner's criteria: 80% of readings at or above the 250 µm minimum, and no reading below 80% of the minimum (i.e., no reading below 200 µm).
Measured: average 210 µm, with several readings at 190 µm.
- The 190 µm readings fail the absolute floor of 200 µm (80% of 250). Even one such reading disqualifies the strake under the 'no reading below 80% of minimum' rule.
- The average of 210 µm is below the 250 µm minimum, so the '80% at or above minimum' criterion cannot be met either.
Disposition: the strake is non-conforming. It cannot be accepted as-is.
Options, in order of preference:
- Apply an additional coat of the same epoxy to build DFT into the 250–350 µm range, after proper surface preparation (abrasion/solvent wipe per data sheet) and within the recoat window.
- If the film is too far under and adding a coat would exceed the maximum DFT or the system's specified number of coats, strip and reapply.
Before disposition, verify the DFT gauge calibration and that readings were taken on a properly prepared, clean surface with the correct probe and curvature correction - but the 190 µm floor breach is decisive. Document readings, locations, and the corrective action.