5.2 Stripe Coating and Critical Areas

The Stripe Coat Procedure

Quick Answer: Stripe coating is the brush application of a coat to critical areas — welds, edges, corners, fasteners, and crevices — before the full coat is applied over the entire surface. The stripe coat goes on first, by brush (brushing forces coating into irregular geometry and wets the substrate better than spray), and it is typically a hold point in the inspection plan: the inspector verifies the stripe coat is present, continuous, and at specified thickness before the applicator proceeds with the full spray coat. The stripe coat compensates for the physical fact that liquid coatings thin at sharp geometry during cure, leaving edges and weld toes under-coated — and under-coated edges are where corrosion starts.

Why Edges Fail First

The thin DFT risk at edges is a physical phenomenon the CIP inspector must explain. When a liquid coating cures, it shrinks (film contraction). On a flat surface, shrinkage is uniform and the film maintains thickness. At a sharp edge — a 90° corner, a weld toe, a flame-cut edge — surface tension pulls the liquid coating away from the sharp geometry toward the flat faces during cure; the coating draws back, leaving a thinned or bare spot where the coating is most needed. Edges also present less surface area per unit of applied volume than flat faces, so the same wet film thickness produces less dry film at the edge. The combination — surface tension draw-back plus geometric thinning — means edges and weld toes routinely measure 50–70% of the flat-surface DFT if not stripe-coated. In service, edges fail first because they have less coating and because moisture, salts, and oxygen concentrate at geometric features — exactly the conditions that initiate corrosion.

The Stripe Coat Sequence

The stripe coat is applied in a defined sequence, and the inspector verifies each step:

1. Surface preparation complete — the weld, edge, corner, or fastener is prepared to the specified SSPC-SP grade and verified.
2. Stripe coat by brush — the applicator brushes the first coat onto welds, edges, fasteners, and crevices, forcing coating into the weld toe and around the edge profile and building a film that does not draw back during cure.
3. Hold point / inspection — the inspector verifies the stripe coat is present, continuous, and at the specified DFT (or WFT). The product data sheet or specification dictates timing — typically the full coat follows within the same recoat window, or the stripe coat becomes tack-free before overcoating.
4. Full coat over stripe coat and substrate — the full coat is applied over the entire surface, including stripe-coated areas. The stripe coat plus full coat at edges should produce a total DFT roughly equal to the full-coat DFT on the flat surface.

Over-Thickness and Under-Sprayed Coating

Stripe coating solves the under-thickness problem at critical areas, but the inspector must also watch for over-thickness. Applying the stripe coat too thick, or applying the full coat without accounting for the stripe coat already in place, can produce localized DFT that exceeds the maximum. Over-thickness is not "more is better." It causes mud cracking (the film is too thick to cure uniformly and cracks as solvent evaporates), solvent entrapment (trapped solvent causes blisters and pinholes), sagging (gravity pulls the thick wet film down), and reduced adhesion (thick films carry higher internal stress). The specification usually limits stripe coat DFT to the same range as the full coat or slightly above, and the inspector measures the stripe coat DFT separately from the total DFT. Edge DFT measurement is part of SSPC-PA 2 practice.

Under-sprayed coating is a related defect: areas that received insufficient coating because the spray pattern missed them, the gun was too far, the overlap was insufficient, or the applicator skipped a pass. Under-sprayed areas appear as thin spots, holidays, or bare substrate, detected by visual examination, WFT comb gauge measurement, and DFT measurement after cure. Under-spray at edges is common because spray guns feather at the pattern edge and a 90° corner presents a changing angle to the fan — exactly why stripe coating is required first.

Critical Areas Checklist

The specification and standard drawings define which areas are critical:

Inspector Role at the Stripe Coat Hold Point

Stripe coating is usually a hold point in the inspection plan. The inspector confirms the critical areas are identified per the specification, verifies the stripe coat is applied by brush before the full coat, measures the stripe coat DFT against the specification, documents the result on the daily inspection report, and releases the hold point only when the stripe coat meets the specification. Skipping the stripe coat, applying it by spray instead of brush, or applying the full coat before the stripe coat is inspected are non-conformances that must be reported on an NCR. The inspector's authority at a hold point is to stop work until the stripe coat is verified — one of the clearest examples of stop-work authority on a coatings project.