Wet Film Thickness Conflict on a Tank Interior
An applicator is spraying a 250 µm DFT epoxy lining on the interior of a potable water tank. The product data sheet specifies 200–300 µm DFT and a wet film thickness (WFT) of 250–375 µm assuming 80% solids by volume. The inspector measures WFT with a comb gauge at several points and records readings of 180–220 µm. The applicator says the DFT will be fine because the steel is warm and the film will 'flow out.' The specification requires DFT verification by magnetic gauge after cure, and the owner requires a minimum of 200 µm DFT at every reading. Ambient conditions are within the data sheet range.
1. Calculate the expected DFT from the measured WFT and determine whether the applicator's claim is defensible.
Read the worked answer
Calculation: DFT = WFT × % solids by volume (as a fraction).
- At 180 µm WFT: 180 × 0.80 = 144 µm DFT
- At 220 µm WFT: 220 × 0.80 = 176 µm DFT
Both are below the 200 µm minimum DFT. The measured WFT range of 180–220 µm corresponds to a DFT range of roughly 144–176 µm - below specification even at the top of the measured range.
Applicator's claim: Not defensible. 'Flow out' describes leveling of the film, not an increase in solids content; the film cannot gain thickness by flowing. Solvent loss reduces wet thickness during cure, so the cured DFT will be at or below the calculated value, not above it.
Conclusion: The applied film is underthick and must be corrected (additional coat or re-application per the coating manufacturer's guidance) before DFT verification.
2. What should the inspector do next, and how should the WFT and DFT measurements be recorded to be defensible?
Read the worked answer
Immediate action: Stop application in the affected area, notify the applicator and contractor, and document the WFT readings with location and time. Because the film is underthick, the inspector should require corrective action per the manufacturer's recommendation (typically an additional coat after proper surface preparation and within the recoat window).
Recording for defensibility:
- Record WFT gauge type, calibration date, and readings at defined grid points.
- Record the solids-by-volume basis used for the calculation.
- After cure, verify DFT with a magnetic gauge per the specification's method (e.g., SSPC-PA 2 or equivalent), including number of readings per area and acceptance criteria.
- Note ambient conditions at application and at verification.
- Document the nonconformance, the corrective action, and re-verification results.
Key point: WFT is a leading indicator; DFT after cure is the acceptance criterion. Both should be in the report, linked by the solids calculation.
3. The applicator proposes adding thinner to improve flow and increase DFT. Evaluate this proposal against the data sheet and specification.
Read the worked answer
Evaluation: Adding thinner is generally not an acceptable fix for underthick film. Thinners reduce solids content, which lowers DFT for a given WFT - the opposite of the desired effect. Most data sheets specify a maximum thinner amount, and exceeding it can degrade chemical resistance, adhesion, and cure, which is critical for a potable water lining.
Correct approach: Achieve the required DFT by applying additional material at the specified WFT, not by altering the product. If thinning is permitted at all, it must be within the data sheet limit and approved in writing; it should not be used to compensate for low film build.
Inspector's role: Reject the thinning proposal as a thickness remedy, require additional coating per the manufacturer's instructions, and document the decision and rationale. Verify that any product modification is within the data sheet and approved by the owner.