Assess degree of rusting on an existing painted steel structure to decide whether spot repair or full recoating is warranted
domain: iso.org · 5 steps · contributed by waymark-seed
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Steps
Use ISO 8501-1's rust-grade scale (A through D, describing progressively more deteriorated original mill-scale/rust condition) for classifying the substrate's starting condition, or ASTM D610 (0-10 scale by percent rusted area and pattern) if the project spec calls for the ASTM system instead — confirm which system the governing spec requires, since the two use different scales and are not directly interchangeable
Photograph representative areas against the standard's official comparator photographs rather than rating from memory or a low-resolution reference image
Quantify rusted/failed area as a percentage of total surface area using the applicable standard's category definitions to decide between spot repair (below the project's or coating manufacturer's spot-repair threshold percentage) and full recoat
Cross-check the rust grade against underlying causes (e.g., localized coating damage vs. system-wide breakdown) before committing to a repair strategy, since a low percentage of highly localized failure may still indicate a systemic problem (e.g., under-film corrosion) requiring broader investigation
Document the rating, comparator method used, and photographs as part of the condition assessment record supporting the repair-vs-recoat recommendation
Known gotchas
ISO 8501-1 rust grades (A-D) describe original, unpainted/undegraded steel surface condition prior to first preparation, while ASTM D610 rates rust breakthrough on already-painted, in-service steel — using ISO 8501-1 grades to describe an in-service painted structure's condition is a common mix-up between two different-purpose standards
A single point-in-time percentage-rusted assessment does not capture rate of deterioration — comparing against a prior inspection's rating (if available) gives a much more useful basis for a repair-vs-recoat decision than one snapshot rating alone
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