Stripe Coating
Why edges, welds, bolts and crevices need an extra coat of their own — and how to apply a stripe coat that actually adds protection where coatings fail first.
Key takeaways
- A stripe coat is an additional coat applied only to edges, welds, corners, fasteners and other irregular areas before or between full coats.
- These areas receive less coating from spray because of surface tension and fan geometry, and they are where corrosion usually starts.
- Stripe coats are most often brushed, which works material into crevices and around fasteners; some specifications allow spray striping.
- Stripe coats must respect the coating’s recoat window and should be a contrasting color so coverage can be inspected.
Look at an aging steel structure and the first rust rarely appears in the middle of a flat plate. It shows along flange edges, at weld seams, around bolt heads and between back-to-back angles. These areas are hard to coat, easy to miss and naturally receive less film. Stripe coating is the practical answer: an extra, deliberate coat applied to exactly those places.
Stripe coating is required by many specifications for immersion linings, marine structures and steel in aggressive environments, and it is considered good practice on almost any protective coating system.
Why edges and welds need extra coating
Liquid coatings pull away from sharp edges as they flow and cure. Surface tension draws the wet film back from the corner toward the flat faces, so a coating applied at the specified thickness on the plate may be only a fraction of that thickness on the edge. This is the main mechanism behind edge corrosion.
Spray geometry adds to the problem. A spray fan hits an edge obliquely, so less material lands there, and the narrow surface is easily missed. Welds add rough, porous surfaces, spatter and undercut that trap air and resist wetting. Crevices between plates or around fasteners are shadowed from the spray altogether, and once corrosion starts inside them it can be difficult to stop — see pitting and crevice corrosion.
Stripe coating works together with good fabrication detailing. Rounding or chamfering sharp edges and smoothing welds during edge and weld preparation reduces film pull-back; the stripe coat then makes up the remaining deficit.
Where to apply a stripe coat
| Feature | Why it is vulnerable | Striping approach |
|---|---|---|
| Plate and flange edges | Film pull-back; impact damage | Brush along and around the edge onto both faces |
| Welds | Rough profile, spatter, undercut, porosity | Work brush across and along the bead |
| Bolts, nuts and washers | Complex geometry; crevices under heads | Dab and work coating into threads and joints |
| Back-to-back angles, lap joints | Crevices shadowed from spray | Brush into the gap as far as practical |
| Cut-outs, scallops and holes | Inside edges, hard access | Brush around the full perimeter |
| Inside corners and stiffener toes | Overspray buildup, poor wetting | Brush to ensure wetting without excessive build |
How to apply a stripe coat
Brushing is the traditional and most common method because the bristles push coating into pores, around fasteners and into crevices in a way spray cannot. Rollers and daubers are sometimes used on long, straight edges. Spray striping with a narrow fan is permitted by some specifications to increase productivity, but it does not work coating into irregularities as effectively and is usually best followed by brushing on welds and fasteners. See brush and roller application for tool selection.
- Confirm surface condition. Make sure the steel or previous coat is clean, dry and within its recoat window.
- Mix correctly. Mix and induct the coating per the product data sheet; mix small batches if pot life is short.
- Brush edges first. Apply along each edge, extending onto both adjacent faces by roughly 1 in (25 mm) or the width the specification requires.
- Work welds and fasteners. Brush across welds and around every bolt, nut and washer, working material into threads and crevices.
- Avoid excessive build. Apply a full but not heavy coat; overly thick stripe material can crack, sag or trap solvent.
- Allow to dry. Observe the minimum recoat time before the full coat goes on.
Use a stripe color that contrasts with both the substrate and the following coat. Inspectors can then see at a glance whether every edge, weld and bolt has been striped — and later coats will reveal if any stripe was skipped.
Timing and sequence
Specifications differ on when to stripe. Common sequences include:
- Stripe, then full coat — the most common, ensuring edges are covered before the main coat bridges them.
- Full coat, then stripe — sometimes used for primers that must go on immediately after blasting to prevent flash rust; the stripe follows once the primer can be overcoated.
- Stripe before every coat — common for immersion linings, where each coat in the system receives its own stripe.
Whatever the sequence, both the stripe and the following coat must be applied within the coating’s recoat window. A stripe coat that sits too long can lose intercoat adhesion, and one overcoated too soon can trap solvent.
Inspecting stripe coats
Inspection is largely visual: every edge, weld and fastener should show uniform stripe color with no misses or holidays. Dry film thickness on edges is difficult to measure with standard gauges, so most specifications rely on visual verification of stripe coverage combined with DFT measurement on adjacent flat surfaces. Holiday testing of linings, where specified, will often reveal missed or thin stripe areas.
Advantages
- Adds film where corrosion usually starts
- Brushing works coating into crevices and around fasteners
- Improves holiday-free results on linings
- Low equipment cost
Limitations
- Labor-intensive, especially on complex steelwork
- Adds a recoat interval to the schedule
- Edge DFT is hard to verify by measurement
- Too much stripe build can cause cracking or solvent entrapment
Frequently asked questions
Is stripe coating the same as a full coat?
No. A stripe coat covers only edges, welds, fasteners and similar features. It is in addition to the full coats in the system, not a replacement for one.
Can stripe coats be sprayed?
Some specifications allow it, often with a narrow fan, but brushing is usually preferred or required on welds, fasteners and crevices because it works the coating into irregular surfaces.
What coating is used for stripe coats?
Normally the same product as the following coat, sometimes tinted to a contrasting color. Follow the specification and the manufacturer’s recommendations.
Does the stripe coat count toward total DFT?
Stripe coats add thickness only on edges and features. Specified DFT is normally measured on flat areas, so the stripe is considered extra protection rather than part of the measured system thickness.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
