Why edges need extra coating
When a liquid coating dries, surface tension pulls it away from sharp edges. The film on an edge ends up thinner than on the adjacent flat surface, sometimes much thinner. Welds, bolt heads, nuts, back-to-back angles and pitted areas have the same problem, plus shadow zones that spray does not reach well. These details are where rust usually appears first on a coated structure, even when the flat areas meet the specified dry film thickness.
Where a stripe coat is applied
- flame-cut and sheared edges, corners and plate ends;
- welds, weld spatter zones and stitch-welded joints;
- bolts, nuts, washers and rivets;
- gaps, lap joints and other crevice areas;
- cut-outs, holes, brackets and complex shapes;
- pitted or uneven areas left after surface preparation.
How it fits the sequence
| Option | Description | Points to watch |
|---|---|---|
| Stripe before the full coat | Brush the details first, then spray the full coat | Respect the minimum time before overcoating |
| Stripe after the full coat | Spray first, then work the details | Easier to see the details that need attention |
| Stripe with each coat | Repeat striping with primer and intermediate coats | Common in severe exposures and immersion |
The specification states which coats are striped and in which order. A contrasting colour for the stripe coat helps inspection confirm that every detail has been covered. The brush is preferred because it works the coating into crevices and around profiles.
Related design measures
Edge rounding before preparation reduces the problem at its source. ISO 12944-3 covers design features that make coating easier, including edges, welds and inaccessible areas, and ISO 8501-3 sets preparation grades for welds and edges. Stripe coating complements these measures rather than replacing them.
Inspection
Inspectors check that stripe coats were applied, look for missed details and confirm continuity. Film thickness on edges is hard to measure directly, so visual checks and, where specified, a holiday test are used.
Stripe coats are also used on repaired areas during maintenance painting, where the edges of sound old coating meet newly prepared steel.
Stripe coating is standard practice in our steel structure painting projects.