A corrugated iron roof takes a hammering in New Zealand. Strong UV, salty coastal air, heavy rain and hard frosts all wear down the factory finish until the roof starts to chalk, fade and, eventually, rust. The good news is that a sound metal roof doesn't have to be replaced just because it looks tired. With the right preparation and the right metal roof paint, you can restore it yourself and add years to its life.
This guide walks you through painting a galvanised, Colorsteel or rusty corrugated iron roof the way the professionals do it. Speciality Coatings is a New Zealand manufacturer based in Hamilton, Waikato, making coatings for New Zealand conditions, and everything here follows our own S300 application manuals.
When does a metal roof need painting?
Walk around the house and look up. Your roof is due for a repaint if you see:
- Chalking: the colour rubs off on your hand as a fine powder.
- Faded or patchy colour, especially on north-facing slopes.
- Moss, lichen or mould building up on the sheets.
- Light orange surface rust at laps, fixings, gutters or where the coating is worn.
- Peeling or flaking paint from a previous coating.
Painting is for roofs that are still sound and watertight. If the roof is leaking, or the steel has rusted right through, paint won't fix it. Replace badly rusted sheets first, and use a waterproofing system such as S400 (expected life 10 years) for leaks, cracks and movement joints.
Tools and safety
You'll need:
- Ladder, harness and roof anchor or edge protection, plus soft-soled shoes with good grip.
- Pressure washer, wire brush, scraper or abrasive pad, and a screwdriver for probing rust.
- Muckoff Organic Moss & Mould Killer and a sprayer.
- Rust Converter for rusty areas, and Super Etch Primer for bare galvanised steel or Zincalume.
- S300 Roof Coating System, a roller and roller tray, a brush for edges and laps, or airless spray gear.
- Gloves, safety glasses and old clothes.
Working at height is dangerous. Falls from roofs are a leading cause of serious injury on New Zealand building sites. Plan how you'll stop a fall before you climb up, never work on a wet or dewy roof, walk only along the purlin lines where the sheets are fixed, and keep someone on the ground. If you're not confident, hire a professional. See WorkSafe New Zealand guidance on working at height for more.
Pick the right weather
Paint only goes on a clean, completely dry roof. Choose a settled spell with no rain forecast until the coating has dried and cured. Don't apply S300 below 10°C or above 35°C. In most of New Zealand that makes late spring to early autumn the best time.
Start once the morning dew has fully dried off the sheets, and finish early enough in the afternoon for the coating to dry before the evening dew settles. S300 is touch dry in about 2 hours. In summer, iron roofs get very hot in the midday sun, so work on the cooler side of the roof, or in the morning and late afternoon, and avoid coating sheets that are too hot to touch.
On a job that runs over several days, it can pay to start the next session where you finished last time and work your way backwards. That way you can check the previous day's work in fresh light and make sure you don't repeat any mistakes.
How to paint a corrugated iron roof, step by step
1. Inspect and repair
Walk the whole roof. Replace loose or failed screws and nails, repair damaged flashings, and scrape away loose or flaking paint. Probe rusty spots with a screwdriver: if the steel flexes, breaks away or has holes, replace that section.
2. Remove loose rust
Wire brush, scrape or sand off loose rust and flakes. You don't need to grind back to bright steel; remove only loose material so the Rust Converter can reach solid rust.
3. Clean with Muckoff
Mix Muckoff 1 part to 5 parts water for a fast clean, or 1 to 20 for a slower clean with minimal scrubbing. Let it kill moss, mould and lichen and lift dirt and chalking, then pressure wash thoroughly. Let the roof dry completely. Most professionals spend more time cleaning than painting.
4. Treat rust with Rust Converter
Brush, roll or spray Rust Converter over all prepared rusty areas. Red rust turns black in about 30 minutes, and in 2–4 hours it has cured into a black primer. No extra primer is needed: S300 goes straight on.
5. Prime bare galvanised steel or Zincalume
S300 is self-priming on most sound roofs, including well-bonded Colorsteel. Bare galvanised steel and Zincalume need Super Etch Primer first. Abrade the metal clean, apply one coat, and overcoat with S300 within 24 hours.
6. First coat of S300
Cut in laps, ridges and around fixings with a brush, then roll along the corrugations, keeping a wet edge to avoid lap marks. Apply an even film; don't stretch the paint past its coverage.
7. Second coat of S300
S300 is touch dry in about 2 hours and can be recoated after 4 hours. Put the second coat on within 3 days of the first. If you leave it longer, you'll need to clean the roof again before recoating. Apply the second coat at 90° to the first. Two full coats are the minimum for the S300 system.
8. Inspect and clean up
Check the roof from several angles in good daylight and touch up thin spots or misses. S300 is water-based: wash rollers and brushes with soap and water straight away.
How to paint a metal roof with a roller
A long-nap roller on an extension pole works the paint into the corrugations. Load it well, roll down the length of the sheet, and finish each sheet before moving on. S300 can also be brushed or sprayed, and no thinning is required.
How much paint do you need?
Measure the roof length times width for each plane and add them up. S300 covers roughly 7.3 m² per litre per coat, so a 15L bucket does about 110 m² per coat. Always allow for two coats.
| Worked example: 120 m² roof | Amount |
|---|---|
| Area to coat (2 coats) | 240 m² |
| S300 needed at about 7.3 m²/L | about 33 L |
| Order option A | 2 × 15L + 1 × 4L (34 L) |
| Order option B | 1 × 15L + 2 × 10L (35 L) |
| Rust Converter (8–10 m²/L), 15 m² of rusty patches | 2 L |
Rough, rusty or porous sheets use more paint, so round up. S300 is sold in test pots, 4L, 10L and 15L. Colours are Gull Grey, Grey Friars, Ironsand, Flaxpod, New Denim Blue and Ebony: shades similar to colours familiar on New Zealand roofs, though not exact matches to any other brand's colours, so grab a test pot to check against your roof first.
DIY or hire a pro?
DIY suits you if:
- The roof is low-pitched, single storey and safe to walk.
- Rust is light surface rust only.
- You have safe access and fall protection.
Call a professional if:
- The roof is steep, high or fragile.
- Sheets are rusted through or the roof leaks.
- It's a commercial or complex project.
For bigger or more complex jobs, our network of approved applicators can help, and our specialist membrane systems are available only through them.
Frequently asked questions
What is the best paint for a corrugated iron roof?
A flexible, UV-resistant coating made for metal roofs. S300 is a water-based acrylic elastomeric roof coating made in New Zealand, with about 280% elongation, so it moves with the roof as it heats and cools. Two coats over a clean, prepared roof give an expected life of 10–15 years.
How do I paint a rusty corrugated iron roof?
Remove loose rust, clean with Muckoff, treat remaining surface rust with Rust Converter, let it cure for 2–4 hours into a black primer, then apply two coats of S300. Replace any sheets rusted through.
Do I need a rust converter for my roof?
Yes, wherever there's surface rust. Rust Converter stabilises it, stops corrosion spreading under the new paint and acts as the primer. It can't repair steel that has rusted through.
Can I repaint a Colorsteel roof?
Yes. As long as the existing finish is well bonded, clean it with Muckoff, let it dry and apply two coats of S300, which is self-priming on sound Colorsteel.
What is the best paint for a rusted galvanised metal roof?
Treat the rust with Rust Converter, prime any bare galvanised steel or Zincalume with Super Etch Primer, then finish with two coats of S300 roof paint.
Get started
Ready to restore your roof? Browse our roof paint range, read the application manuals and data sheets, or contact us: call 027 702 2227 or email info@specialitycoatings.co.nz for advice on your roof.