How to Clean Colorbond Fencing and Roofing (the BlueScope Way)
Last spring I got called to a place in Marrickville where the owner had pressure-washed his new Colorbond fence at 3000 psi the previous Saturday. By the Friday it was raining, and by Monday the paint had lifted off in sheets where the water had got under the polymer coating. The whole fence had to be replaced. Right, gear first — and a quick word on what Colorbond actually is, because half the cleaning advice online is wrong. It’s a BlueScope product with a baked-on, oven-cured polymer paint over a metallic conversion layer. It’s not American “metal siding”. Treat it like American siding (blast it with a pressure washer) and you actively void the 25-year warranty. Here’s the science of why the gentle method is the only correct method, and how to do it in an hour without damaging the surface.
Gear you’ll need
- Long-handled soft-bristle car wash broom — Sabco from Bunnings, $18 — NOT a stiff deck brush
- Garden hose with a normal trigger nozzle — no pressure washer
- A mild liquid dishwashing detergent — Morning Fresh, Palmolive, Earth Choice or supermarket own-brand
- A bucket of warm water (about 9 litres)
- For tougher dirt: 1 cup of household ammonia in 4L water (separate bucket)
- Clean white microfibre cloths for spot work
- Step ladder or extension pole for high panels and gutter strips
- Rubber gloves and eye protection
- Drop sheet over plants if you’re using ammonia
Step 1: Read the BlueScope care guide first (the gotcha)
BlueScope publishes a free care guide on their website. The two non-negotiables: no abrasive cleaners (no Jif, no Gumption, no Scotchbrite pads) and no pressure washing above 1500 psi, and even at 1500 psi only with a 25-degree fan tip held 600mm back from the surface. Any harsher and you void the 25-year warranty. Most consumer pressure washers from Bunnings or Kmart are 2000-3000 psi out of the box — you cannot use them on Colorbond, full stop. The polymer coating is roughly 25 microns thick. A pressure-washer jet at 3000 psi will hairline-crack that coating and the warranty is gone.
Step 2: Hose the surface down first
Before any detergent, rinse the whole panel or roof section with the garden hose on a normal trigger nozzle. This removes loose dust, cobwebs, dead bugs and bird droppings. Skip this step and you grind grit into the polymer with the broom — which is exactly the abrasion BlueScope warns against. Two minutes of pre-rinse saves you the warranty.
Step 3: Mix mild soap solution
Half a teaspoon of dishwashing liquid in 9 litres of warm water — a standard household bucket. That’s all. More soap doesn’t clean better; it just leaves residue that dries chalky and attracts dirt within a fortnight. Here’s the science — dishwashing liquid contains surfactants that reduce water’s surface tension and let it lift dirt off a hydrophobic painted surface. You don’t need scour. You need surfactant plus dwell time. Skip the fancy spray, just use dish soap.
Step 4: Soft wash from the bottom up
Counter-intuitive, but on a vertical surface (Colorbond fence panels) you wash bottom-up. Why: if you wash top-down, dirty soapy water runs down onto dry, untreated panel below and stains it — especially noticeable on chalky colours like Surfmist or Classic Cream. Bottom-up keeps the dirty water flowing onto already-wet, already-soaped panel where it rinses clean. For Colorbond roofing, top-down is fine because gravity is doing the work and you’ll be on a ladder — just rinse thoroughly afterward.
Step 5: Use the soft broom in straight strokes
Long parallel strokes, in line with the panel’s profile — vertical strokes on a fence panel with vertical ribs, along the ridge on roofing. Don’t scrub in circles. Circular motion polishes some areas brighter than others and the difference is visible in raking afternoon light forever. Light pressure only. The detergent is doing the work, not your shoulder. If you find yourself pressing, give the soap another 30 seconds of dwell time first.
Step 6: Tackle stubborn marks with diluted ammonia
For mould (north-facing panels in shaded gardens), bird poop that’s set, or general grime the soap won’t shift — household ammonia, 1 cup in 4L of water, applied with a soft cloth, left to dwell 30 seconds, then rinsed. Never mix ammonia with bleach — produces chloramine gas, which is toxic. And don’t substitute domestic bleach (sodium hypochlorite) for the ammonia — bleach attacks the polymer over time even if it looks fine the day you apply it. Pine O Clean and similar pine-oil cleaners are also not Colorbond-friendly; stick to the mild dish soap or the ammonia mix.
Step 7: Rinse thoroughly with the hose
Soap residue dries to a chalky film that attracts dirt within a fortnight. Rinse with the hose on jet — normal hose-trigger jet, not pressure-washer jet — until water runs clear off the bottom edge with no foam. Run your hand over a rinsed panel: if it feels slippery, there’s still soap. Re-rinse. This step is where I see alot of DIY jobs go wrong — they soap thoroughly, then rinse for 30 seconds and call it done. The chalky soap residue is then the new “dirt” the surface attracts.
Step 8: Get into the gutter overhang on roofing
On Colorbond roofing, the dirt accumulates at the gutter line because that’s where runoff slows and deposits sediment. Sit on a roof anchor or work off a roof ladder — don’t free-walk a wet Colorbond roof, the soap on top of polymer is genuinely treacherous to walk on. Hand-wash that 300mm strip above the gutter with a microfibre cloth, then rinse downward into the gutter. Clean the gutter while you’re up there — leaves and grit in the gutter wick water back under the polymer edge and that’s how rust starts.
Step 9: Spot-treat tannin and rust marks separately
Tannin from gum leaves sitting on Colorbond leaves rust-coloured stains over time. Don’t try to scrub these off with anything abrasive. Bar Keepers Friend (oxalic acid) on a damp microfibre, dabbed on and rinsed within 60 seconds, lifts tannin without scratching — same product I use on shower glass in our scale-removal tutorial. For actual rust — usually from a steel roof screw bleeding onto the panel — replace the screw with a proper Colorbond-coloured Tek screw, then treat the stain. Don’t paint over rust spots; they bleed through within a year.
Step 10: Schedule it 6-monthly in coastal homes, yearly inland
BlueScope recommends every 6 months within 1km of the coast, every 12 months elsewhere. Salt deposits accumulate fast in coastal Sydney, Newcastle, the Gold Coast and Perth — and salt is a slow corrosive on the polymer edges where the coating is thinnest. Even an unwashed inland Colorbond fence collects enough atmospheric dust in a year to start chalking the surface — and chalking is the precursor to actual paint failure. Twice-a-year is the routine that keeps the 25-year warranty meaningful.
Roof cleaning — the safety story
If you’re cleaning a Colorbond roof, safety matters more than the cleaning method. Wet metal roofing is genuinely slippery — soap on top of polymer is treacherous. Hire a roof harness anchor and a fall-arrest harness from Kennards if you don’t own one, or engage a professional. Soft-wash roof cleaning in Sydney runs $400-700 for an average home, and it’s worth every cent compared to a fall from a single-storey eave height. Double-storey middle-of-roof DIY is not a thing — full stop.
Moss and lichen on the roof — the slow method
South-facing Colorbond roofs in shaded gardens grow moss and lichen, especially in temperate Sydney and Melbourne climates. Don’t scrape it off — you’ll damage the polymer underneath. The trick is a dilute Wet & Forget solution (the green-and-white bottle, 1:5 with water in a garden sprayer), applied on a dry day. Leave it on, don’t rinse — over the next 6-12 weeks of weather cycles the moss/lichen dies and washes off naturally with rain. It’s slow but it’s the only method that doesn’t damage the surface.
When you should NOT DIY this
Anything above single-storey eave height needs a professional with proper roof anchors and a fall-arrest harness — falls from height are the leading cause of DIY fatalities in Australia and Colorbond roofs are genuinely slippery wet. Paint overspray, fire smoke residue or chemical staining (someone’s pool acid splash, for instance) needs a specialist, not a soap-and-broom job. And if the polymer has visibly lifted, cracked or chalked badly, no cleaning fixes that — it’s a paint repair or panel replacement, and most insurers will only cover it if the warranty trail is clean (i.e. you used BlueScope-approved methods).
Common screw-ups
- Pressure-washing — the single fastest way to void a Colorbond warranty.
- Using Jif, Gumption or any scouring cream — leaves micro-scratches that hold dirt and start the chalking process early.
- Mixing ammonia with bleach — chloramine gas, genuinely dangerous.
- Soap-then-skip-the-rinse — chalky residue becomes the next layer of dirt within two weeks.
- Free-walking a wet Colorbond roof in runners — the way most DIY falls happen.
Cost & time
Gear: about $25 for the broom, soap, ammonia and gloves if starting from nothing. Time: 60-90 minutes for an average suburban fence or single-storey roof perimeter. Compared to professional Colorbond cleaning at $400-700 for a house, doing it yourself twice a year saves you $1000+ annually if you can do it safely.
Soft broom, mild detergent, gentle pressure, hose rinse — that’s it. The pressure washer stays in the shed for the concrete driveway and the car wheels. The 25-year warranty is worth more than the half-hour you save by blasting it. And every “metal siding” article scraped from US sites is wrong for Aussie Colorbond — different paint chemistry, different rules. While you’re outside cleaning, the gutters on most homes are the next 30-minute job worth doing on the same Saturday — leaves rotting in there are the source of half the rust streaks that appear on Colorbond fences below. Caddy out, let’s go.
