Concept · Conditions and application

Stripe coats

Why edges, welds, corners and bolts get an extra brushed coat, and how inspectors check it.

A stripe coat is a band of paint brushed or sprayed by hand over the details of a structure, such as edges, welds, bolts and corners, in addition to the normal coats. It usually goes on before the full coat. Spray leaves less film on these details. They are also the places where coatings fail first.

The why

Wet paint is a liquid, and a liquid pulls itself into the smallest surface it can. On a flat plate this makes a smooth film. A sharp edge has almost no flat area. The wet film cannot hold a thick layer on the point, so surface tension pulls it back and it runs onto the flat faces.

The film then dries and shrinks as the solvent leaves. This shrinking pulls the film away from the edge again. The result is a thin film at the point where the steel is most exposed.

Other details have their own problem. Spray cannot reach the shadow behind a bolt, a bracket or a stiffener. In heavy pitting, spray bridges the pits and leaves air in them. A brush wets the steel better, and it works the paint into pits and crevices.

Edge preparation helps. A rounded edge holds more film than a sharp one. ISO 8501-3 grade P3 asks for a radius of at least 2 mm, or a chamfer made in three passes. ISO 12944-3 asks for P3 for high and very high durability in C4 and above, and for Im1 to Im4. The stripe coat adds film on top of this work. It does not replace it.

In the field

1. Read the specification for the details to stripe and for the point in the sequence.
2. Check that the edges were prepared as specified before coating, for example rounded.
3. Check that the stripe coat goes on before each full coat or between coats, as the specification says.
4. Watch the applicator work the paint in by brush. A roller is used on some details.
5. Check that every specified detail is covered: edges, welds, corners, cut-outs, bolts and back faces.
6. Check that the colour contrasts with the coat below, so that you see complete coverage. The next coat then hides the stripe.
7. Look for runs, sags and thin spots, and respect the recoat time (drying, curing and recoat windows).

A stripe coat is usually not measured with a DFT gauge, because the irregular shapes give poor readings (see dry film thickness measurement). Judge it by sight: coverage, colour and finish.

More film is not always better. A thick stripe under a full coat can push the total over the maximum DFT of the data sheet, so keep the stripe within the maker's limits and avoid excessive film in corners and cavities. The stripe coat does not replace the full coat or reduce its DFT.

Worked example

A girder is specified at 240 µm. The flat web reads 245 µm. The flange edge reads 120 µm.

Check: 120 ÷ 240 = 0.5, so the edge has half the specified film. A stripe coat on the edges before the full coat prevents this on the next girders. A thicker coat on the web does not help, because it adds nothing at the edge.

Ballast tanks under IMO PSPC

PSPC for dedicated seawater ballast tanks asks for at least two stripe coats and two spray coats. The second stripe coat may be reduced on welded seams only, where the NDFT is proven. The total NDFT is 320 µm. Stripe coats go on by brush or roller. A roller is used on scallops and rat holes only. PSPC also asks for edges with a radius of at least 2 mm.

A missing second stripe coat on a free edge is a nonconformance, even when the spray coats reach the NDFT. The 90/10 rule covers thickness. It does not cover the number of coats.

What the inspector records

Record each detail striped, the date and the product, the application method, the colour and the point in the sequence. Record it apart from the full coat. A missing stripe coat is an NCR, also when the overall DFT is met. The ITP may mark the stripe coat as a witness point (hold points, witness points and the ITP). If the contractor gave the notice and you did not attend, the work may go on and the record notes both facts.

Exam traps

• Trap: spray thins the film at edges because of poor technique. Surface tension and shrinkage do it, even with good spraying.
• Trap: a thicker coat on the flat faces fixes the edges. It does not.
• Trap: the stripe coat replaces part of the full coat. It is extra, and it does not reduce the full coat DFT.
• Trap: a stripe coat goes on last, over the topcoat. It goes on before the full coats or between them.
• Trap: one stripe coat on edges meets PSPC. PSPC asks for two on edges.
• Trap: the 90/10 rule checks the stripe coats. It checks thickness only.
• Trap: the stripe coat must match the colour of the coat below. A contrasting colour shows coverage.

35 practice items in the members area mention this topic (CIP 1: 16 · CIP 2: 4 · FROSIO written: 15).

Related

• Standards: SSPC-PA 1 · NACE SP0178 · IMO PSPC
• Defects: Rust-through (rusting) · Undercutting (underfilm corrosion) · Holidays and misses · Edge corrosion (thin film at edges) · Steel surface imperfections
• Conditions and application: Dew point and the 3 °C margin · Volume solids, wet film and dry film · Pot life and induction time · Drying, curing and recoat windows