Why does corrugated metal need more paint?

Corrugated metal sheets have approximately 35% more actual surface area than their flat projected dimensions. The sinusoidal profile creates peaks and troughs that increase the paintable area. A sheet measuring 2m × 1m flat has about 2.7 m² of actual surface. Structural steel with flanges and webs can have up to 2× the projected area.

What primer should I use on bare metal?

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.

How many coats of paint does metalwork need?

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.

Can I paint over rust?

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.

What is the best paint for external metalwork?

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.

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Metal Paint Calculator — Primer, Undercoat & Topcoat

Calculate paint quantities for metal surfaces including zinc primer, undercoat, and topcoat. Adjusts for surface profile (smooth, corrugated, structural).

Measured Surface Area

Flat projected area

Surface Profile

Adjusts for extra surface area

Number of Topcoats

2 coats standard

Price Per Litre (optional)

How We Calculate This

This calculator determines paint quantities for metal surfaces, adjusting for the surface profile which increases the actual paintable area compared to flat dimensions.

The formula

Actual area = Measured area × Profile factor

Zinc primer = Actual area ÷ 8 m²/L × Wastage

Undercoat = Actual area ÷ 12 m²/L × Wastage

Topcoat = (Actual area × Coats) ÷ 14 m²/L × Wastage

For example, 20 m² of corrugated sheeting (factor 1.35): actual area = 27 m². Primer: 27 ÷ 8 = 3.4 L. Undercoat: 27 ÷ 12 = 2.3 L. Topcoat: 27 × 2 ÷ 14 = 3.9 L. Total: 9.5 L + 10% = 10.5 L.

Profile factors

Smooth flat: 1.0 — no adjustment needed
Light profile: 1.15 — box sections, angles
Corrugated: 1.35 — sinusoidal sheet profile
Structural steel: 1.5 — I-beams, H-sections, channels
Lattice / open steel: 2.0 — trusses, grilles

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.