For keying a metal roof before painting, 80 to 120 grit sandpaper is the most widely recommended range among NZ roof painters. Coarser grits (around 80) tackle rust, old paint, and stubborn mill scale, while finer grits (100–120) create the light, even scratch profile that helps primer and topcoat bond properly without leaving deep gouges. Getting this step right is arguably the most important part of a long-lasting roof paint job.
Why Keying Matters on a Metal Roof
Metal roofing — whether it's Zincalume, Colorsteel, galvanised iron, or old corrugated iron — has a naturally smooth or oxidised surface that paint struggles to grip. Keying (also called scuff-sanding or abrading) creates microscopic scratches that give the primer something to mechanically bond to. Skip this step and even the best paint system can peel within a season or two, especially in NZ's harsh UV and coastal environments.
Choosing the Right Grit
60–80 Grit — Heavy Preparation
- Use this range when dealing with heavy rust, flaking paint, or thick mill scale.
- It removes material quickly but leaves deeper scratches, so always follow up with a finer grit or a good-quality etching primer to fill the profile.
- Avoid going coarser than 60 on thin sheet metal — you risk cutting through protective coatings or even the metal itself.
80–120 Grit — General Keying (Most Common)
- This is the sweet spot for most NZ metal roof prep work.
- 80 grit handles moderate rust and old chalky paint; 100–120 grit is ideal for lightly weathered or previously painted surfaces in reasonable condition.
- Leaves a scratch profile that suits most water-based and oil-based metal primers.
150–180 Grit — Light Scuffing Only
- Suitable for sound, well-adhered existing paint that just needs a light key before a topcoat.
- Not aggressive enough for bare metal, rust, or slick galvanised surfaces.
- Sometimes used as a final pass after coarser sanding to smooth out scratch marks before priming.
Abrasive Format: Paper, Disc, or Pad?
| Format | Best For | Notes |
|---|---|---|
| Sanding sheets / rolls | Hand sanding flat runs | Wrap around a sanding block for even pressure |
| Flap discs (angle grinder) | Rust removal, large areas | Fast but aggressive — use with care on thin sheet |
| Scotch-Brite / abrasive pads | Light scuffing, detail areas | Equivalent to approx. 150–180 grit |
| Random orbital sander | Flat or gently curved panels | Good control, reduces arm fatigue |
For corrugated profiles, hand sanding with a flexible sheet often works better than power tools because you can follow the contours. Flap discs on an angle grinder are efficient for flat areas or heavy rust but require a steady hand — it's easy to cut through a zinc coating if you linger too long.
Surface-Specific Tips
- Galvanised iron: New galvanising has a slick, oily surface. Wash with a galvanised iron cleaner or diluted phosphoric acid wash first, then sand with 100–120 grit. Some painters skip sanding on new galv and rely solely on an etch primer — check your primer manufacturer's datasheet.
- Colorsteel / pre-painted steel: Lightly scuff with 120–150 grit to dull the gloss. Avoid cutting through the factory coating unless it's failing.
- Rusted areas: Remove loose rust with 60–80 grit, treat with a rust converter if needed, then prime promptly — bare metal flashes rust quickly in humid NZ conditions.
- Aluminium: Use 100–120 grit and an aluminium-compatible etch primer. Aluminium is soft, so avoid very coarse grits.
Primer Selection After Sanding
Sanding alone isn't enough — always follow with a metal primer suited to your substrate. Resene and Dulux both offer metal roof primer systems with specific datasheets that confirm recommended surface profiles. Wattyl also has roof coating systems worth considering. Always read the product datasheet — it will specify the minimum and maximum surface profile (often expressed in microns or grit range) for the primer to perform correctly.
Safety on the Roof
Working on a roof carries serious fall risk. WorkSafe NZ requires that anyone working at height — including homeowners doing DIY — takes practicable steps to prevent falls. At minimum:
- Use a roof ladder or crawl board to spread your weight and avoid slipping.
- Never work on a wet or dew-covered roof.
- Wear a dust mask (P2 rated) when sanding — old paint may contain lead on pre-1980s roofs.
- Safety glasses and gloves are essential when using power tools or angle grinders.
If your roof pitch is steep or the surface is fragile, seriously consider leaving the work to a professional.
When to Call a Professional
Roof painting looks straightforward but involves real risks and technical decisions. If your roof has widespread rust, unknown coating history, or is particularly steep, it's worth getting a qualified roof painter involved. A professional will assess the substrate, choose the right prep method, and apply a paint system that meets manufacturer warranties. You can find experienced local roof painters on tradedirectory.co.nz. For guidance on paint systems and durability, BRANZ publishes research and good practice guides relevant to NZ conditions.
Master Painters NZ member
Licensed Building Practitioner (LBP)
Roofing Association of NZ (RANZ)
