Sprayed film is thinnest exactly where corrosion starts. Why welds, cut edges and cut-outs need a hand-applied stripe coat, and how to specify it.
Look at a painted steel structure five years after handover and the failures are rarely in the middle of a plate. They sit on weld toes, cut edges, the inside of a stiffener, the rim of a handrail, the corner of a bracket — the places where the film was thinnest on the day it was applied. That is not bad luck. It is the predictable result of spraying a coating onto a shape with corners.
Why edges end up thin
- Spray angle. A gun works best at 90° to the surface. At a weld toe or a cut edge the angle is oblique, so less material arrives per pass — often 30–50% less on the edge face itself.
- Surface tension. Wet paint pulls back from a sharp corner. A low-viscosity coat flows away from the edge and leaves a thinned band on both sides of it.
- Shadowing. Stiffeners, backing bars, gussets and bolt heads hide the surface behind them. Spray cannot reach what the nozzle cannot see.
- Pass count. High-build products normally go on in one or two passes on flat plate. Nobody adds a third for a 20 mm weld toe unless the specification asks for it.
What a stripe coat is — and is not
A stripe coat is a hand-applied layer on welds, cut edges, corners, cut-outs, bolt heads, backing bars and other areas that are hard to spray, applied before or after the full coat of the same product. It corrects thickness; it is not decoration, and it is not a substitute for the total dry film thickness specified on flat areas. Two things it is not: it is not an extra coat with its own nominal thickness to be added to the system, and it is not a cosmetic touch-up at the end of the job. It is part of the coating sequence and belongs in the inspection plan.
Sequence and timing
- After the first full coat, not before. Striping bare steel means the stripe material bonds to a substrate that the full coat will later cover, which complicates the intercoat relationship for no gain.
- Inside the recoat window. Once the first coat has passed its overcoating interval, a stripe applied on top is a weak interface, not a repair. If the window has been missed, the surface has to be prepared again.
- Mind the product. Zinc-rich primers cure hard and fast; a stripe coat applied too late can lose intercoat adhesion. High-build epoxies left too long become difficult to wet. Both cases are covered by the recoat table in the TDS, not by a rule of thumb.
- Reverse striping for thick systems. On high-build and glass-flake systems, striping after the intermediate coat as well as after the primer is common, so that the edge of the first stripe is itself protected.
Materials and tools
On most systems the stripe coat uses the same product as the coat it sits on, which keeps the chemistry consistent and removes a compatibility question. Zinc-rich primers such as Interzinc 52HS are commonly brushed over welds; where a thicker edge build is wanted, high-build epoxies such as Intergard 475HS and surface-tolerant mastics such as Jotamastic 90 are used the same way. A brush reaches into a weld toe and works paint into the profile in a way a roller cannot; a roller is faster but tends to bridge over the crevice. On visible structures, a contrasting tint in the stripe makes it possible to see from several metres away whether the work was actually done.
Specifying and inspecting it
Stripe coating disappears from projects for one reason: it is described in the method statement but not in the specification, so it is the first item cut when the schedule slips. Put it in the system sheet with three things — which areas, which product, which stage. Inspection is then mostly visual: 100% of welds and cut edges before the next full coat, and again at handover. Dry film thickness on flat plate is measured to ISO 19840, and that standard's guidance on keeping gauge readings away from edges and weld spatter explains why a magnetic gauge will not tell you whether the stripe itself is thick enough. The stripe is verified by seeing it, and by the holiday test on the finished system where one is specified.
A stripe coat costs perhaps 5–10% of the painting labour on a structural job, and a few minutes per metre of weld. The repair it prevents — cutting back a failed edge, re-blasting a stiffener, re-coating a handrail — costs a multiple of that, usually in a place that is hard to reach a second time. ISO 12944 and the IMO PSPC rules for water ballast tanks both treat stripe coating as part of good practice rather than an optional extra, and the specification should say the same. Specific product parameters, including recoat intervals and maximum film thickness, are those in the latest manufacturer TDS — publicly available manufacturer materials are the reference, and they are updated more often than project specifications.