Wet Film to Dry Film Thickness on a Complex Geometry
An inspector is checking a high-build epoxy applied to a fabricated steel structure with stiffeners, bolt heads, and inside corners. The specified dry film thickness (DFT) is 8–12 mils (200–300 µm). The applicator's wet film gauge readings on flat areas average 14 mils (350 µm) wet. The coating is 70% volume solids. On the complex geometry, the applicator reports 'it looks good' but has no wet-film readings. After cure, magnetic DFT gauge readings on flat areas average 11 mils, but inside corners read 18–22 mils and some bolt heads read 5–6 mils.
1. Calculate the expected DFT from the 14 mil wet reading and compare to the specification.
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DFT = WFT × volume solids fraction
DFT = 14 mils × 0.70 = 9.8 mils (≈ 245 µm), which is within the 8–12 mil spec on flat areas. The cured-gauge average of 11 mils is consistent (within gauge and application variability).
So the flat-area application is acceptable. The problem lies elsewhere - the geometry. This calculation shows the applicator's flat-area technique is fine; the failure is distribution, not total volume.
2. Evaluate the corner and bolt-head readings against the spec and explain the mechanism causing each.
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- Inside corners: 18–22 mils - far above the 12-mil maximum. Mechanism: overspray accumulation and 'shadowing' - spray bounces into corners and successive passes build up. Excessive DFT in corners risks mud-cracking, solvent entrapment, poor cure, and internal stress.
- Bolt heads: 5–6 mils - below the 8-mil minimum. Mechanism: spray angle and turbulence around protrusions - the air stream deflects around the fastener, and edges/crevices are hard to cover; also 'dry spray' and shadowing on the lee side.
Both are classic complex-geometry distribution failures: same total paint, wrong distribution.
3. Recommend corrective actions and the inspection approach to prevent recurrence.
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Corrective actions:
- Corners: adjust spray technique (reduce passes, increase distance, angle away from the corner), stripe-coat corners with a brush to control film, and check WFT during application.
- Bolt heads/edges: stripe coat fasteners and edges by brush before or after spray; approach from multiple angles; verify coverage.
Inspection approach:
- Take WFT readings during application at representative points including corners and fasteners, not just flats.
- After cure, take DFT at defined locations (flats, corners, edges, fasteners) - a single flat-area average hides the failures.
- Compare each location to min/max, not just the mean.
- Repair out-of-spec areas (abrade/remove excess in corners if cracked; build up thin areas).
The judgment: 'looks good' is not inspection; thickness must be verified where the geometry makes application hardest.