Structural Steel Painting Calculator — Surface Area & Paint Coverage

Surface area = the section's surface area per metre (m²/m) × the member length (m). The surface area per metre follows the full painted profile of the section — both flanges plus the web, around the root fillets — and is given by Am = 2h + 4b + 2πr − 8r − 2tw (the same exposed perimeter used for fire section factors). The value for each section is published in the Steel for Life 'Blue Book' surface-area-per-metre column. For example, a 254x146x31 UB is 1.06 m²/m, so a 6m beam is about 6.4 m² of painted surface.

DFT (Dry Film Thickness) is the thickness of the paint film after drying, measured in microns (μm). Each coat has a target DFT, and the total system DFT determines the corrosion protection life. A typical system might be: primer 75μm + intermediate 75μm + finish 50μm = 200μm total. Higher DFT gives longer protection but costs more. ISO 12944 defines environment categories and required DFTs.

Sa 2½ (near-white blast) per BS EN ISO 8501-1 is the standard minimum for structural steelwork painting. It removes mill scale, rust, and old coatings, leaving a uniform grey surface with suitable anchor profile. Sa 3 (white metal blast) is used for critical environments or zinc silicate primers. Manual cleaning (St 2/St 3) is only acceptable for minor touch-up or maintenance painting.

For external steelwork in a UK urban/industrial environment (ISO 12944 Category C3-C4): zinc phosphate epoxy primer (75μm) + MIO epoxy intermediate (75-100μm) + polyurethane finish (50-75μm) = 200-250μm total. For marine/coastal (C5): increase to 320μm total with zinc-rich primer. For internal dry (C1-C2): alkyd primer + finish at 125μm may be adequate.

Theoretical coverage (m²/L) = volume solids % × 10 ÷ DFT in microns — a litre of 100% solids paint covers 1000 m² at 1μm, so volume solids is essential. Practical coverage is roughly 60-70% of theoretical to allow for wastage, overspray, surface profile and application losses. For example, an epoxy MIO at ~52% volume solids and 75μm DFT gives a theoretical 6.9 m²/L and a practical ≈ 4-5 m²/L. Always use the volume solids figure from the product data sheet. Multiply surface area by the number of coats, then divide by practical coverage to get the litres needed.