Metal Paint Calculator — Primer, Undercoat & Topcoat

Corrugated metal has more actual surface area than its flat measured dimensions because the peaks and troughs add paintable area. The standard estimating convention (IS 1200, the recognised method of measurement for painting sheet) increases flat-measured corrugated GI/MS sheet by 14% (factor 1.14), and corrugated asbestos-cement by 20%. Deeper box or trapezoidal profiles add more — this calculator offers 1.35 as an allowance for deep-profile sheet, but confirm against your actual profile. Structural steel with flanges and webs can reach roughly 1.5–2× the projected area.

For ferrous metals (steel, iron), use a zinc phosphate or zinc-rich primer for corrosion protection. For aluminium, use an etch primer. For galvanised steel, use a specialist galvanised metal primer (standard primers won't adhere). Red oxide primer is a traditional choice for structural steelwork but zinc phosphate offers better modern performance.

The full system for metal is: 1 coat zinc primer + 1 coat undercoat + 2 coats topcoat. For mild environments, you can reduce to primer + 2 coats topcoat. For harsh or marine environments, use primer + undercoat + 2-3 topcoats. Each layer serves a purpose — primer for adhesion, undercoat for build, topcoat for weather resistance and colour.

Light surface rust can be treated with a rust converter before priming. Heavy rust must be removed by wire brushing, sanding, or shot blasting to Sa 2.5 standard. Products like Hammerite can be applied directly to light rust, but for lasting results on structural steel, proper preparation to bare metal is essential. Surface prep is 80% of a good paint job.

For external metalwork, use a high-performance alkyd or polyurethane topcoat over a zinc-rich primer. Hammerite is popular for DIY but professional contractors use systems like International/Jotun/Hempel for structural work. For gates and railings, a 2-pack epoxy primer with polyurethane topcoat gives excellent durability — expect 10-15 years before maintenance.