Low Film Build on Structural Steel Angles After a Wet Spray
A shop applies a two-coat epoxy system to fabricated structural angles. The specified dry film thickness is 150–200 µm per coat. The applicator sprays with a conventional air spray gun at high fluid pressure and a fast pass, and the inspector measures with a magnetic gauge. On the first batch, readings on flat faces average 165 µm, but readings on the inside corners of the angles and on the edges cluster at 90–120 µm. The applicator says the average is within spec and the low spots are 'just corners.' The owner's specification states that no individual reading may fall below 80% of the specified minimum.
1. Determine whether the coating meets the specification and identify the most likely cause of the low readings.
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Check against the stated acceptance rule: minimum specified DFT is 150 µm, so 80% of minimum is 120 µm. Corner and edge readings of 90–120 µm include values below 120 µm, so the coating fails the owner's individual-reading criterion even though the flat-face average passes. An average does not override a per-reading floor.
Likely cause: spraying geometry. Inside corners and edges are shadowed from the spray path and receive less material; a fast pass with high fluid pressure also reduces deposit and increases overspray. Edges additionally lose coating to runoff and have less area to hold film. The fix is technique and equipment, not more passes at the same settings: reduce fluid pressure, slow the pass, angle the gun into corners, and stripe-coat edges and corners before the full coat.
2. Describe how you would verify the deficiency and what corrective action you would require before the second coat.
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Verification: map the readings by location (flat face, corner, edge) on a sketch, take multiple gauge readings per location, and confirm the gauge is calibrated and suited to the substrate thickness and curvature. Re-measure suspect areas after the film has fully cured, since solvent retention can affect readings. Record the lowest values against the 120 µm floor.
Corrective action before the second coat: the low areas are below the minimum and cannot simply be covered by the next coat if the system requires each coat to meet its own minimum. Require the applicator to abrade and re-coat the deficient corners and edges to bring them to at least 150 µm, stripe-coat edges and corners, and adjust spray parameters. Re-inspect and document the corrected readings, then proceed to the second coat. If the owner allows the second coat to build total thickness, get that in writing before proceeding.