How to Clean Colorbond Fencing and Roofing (the BlueScope Way)

By Priya — deep-clean specialist, Sydney inner-west.

Half the cleaning advice I see online about Colorbond is wrong, and the worst of it comes from US “metal siding” articles that get scraped and republished on Aussie sites. Colorbond is not American steel siding. It’s a BlueScope product with a baked-on polymer coating — specifically, an oven-cured paint system over a metallic conversion layer. Treat it like American siding (i.e. blast it with a high-pressure washer) and you actively void the BlueScope warranty.

I’ve been called to “fix” Colorbond fences in Marrickville and Newtown that the owner had pressure-washed at 3000 psi. The paint had hairline cracked across the panels. Once water gets under the polymer, it lifts in sheets within a year. There’s no fix for that — repainting Colorbond never matches the factory finish and it’s $200 a panel to replace.

The actual BlueScope-approved cleaning method is laughably simple: a soft broom, water, and a mild detergent. That’s it. Here’s the proper routine, the one BlueScope’s own care guide endorses, with the bits they don’t tell you mixed in.

What 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, supermarket own-brand all fine
• A bucket of warm water
• For tougher dirt: 1 cup of household ammonia in 4L water (in a separate bucket)
• Clean white microfibre cloths for spot work
• Step ladder or extension pole for high gutters/roofing
• Rubber gloves, 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 600 mm back from the surface. Any harsher and you void the 25-year warranty. Most consumer pressure washers are 2000-3000 psi out of the box — you cannot use them on Colorbond, full stop.

Step 2: Hose the surface down first

Before any detergent, rinse the whole panel/fence/roof section with the garden hose on a normal nozzle. This removes loose dust, cobwebs, dead bugs and bird droppings. Skipping this step means you grind grit into the polymer with the broom, which is exactly the abrasion BlueScope warns against.

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. The soap is there to break the surface tension, not to scour.

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 in chalky panels like Surfmist). Bottom-up keeps the dirty water always on 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.

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 that has 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 light forever. Light pressure only. The detergent does the work.

Step 6: Tackle stubborn marks with diluted ammonia

For mould (north-facing panels in shaded gardens), bird poop that’s set, or general grime that 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. Don’t mix ammonia with bleach — produces toxic chloramine gas. Ever. And don’t substitute domestic bleach (sodium hypochlorite) for ammonia — bleach attacks the polymer over time even if it looks fine the day you apply it.

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 (not pressure-washer jet — normal hose-trigger jet) until all soap is gone and the water runs clear off the bottom edge. Run your hand over a rinsed panel — if it feels slippery, there’s still soap.

Step 8: Get into the gutter overhang on roofing

On Colorbond roofing, the dirt accumulates at the gutter line. Sit on a roof anchor or work off a roof ladder — don’t free-walk a wet Colorbond roof, the soap on top of the polymer is genuinely dangerous to walk on. Hand-wash that 300 mm strip above the gutter with a microfibre cloth, then rinse.

Step 9: Spot-treat tannin and rust marks separately

Tannin from gum leaves on top of Colorbond leaves rust-coloured stains. 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. 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.

Step 10: Schedule it 6-monthly in coastal homes

BlueScope recommends every 6 months within 1 km of the coast, every 12 months elsewhere. Salt deposits accumulate fast in coastal Sydney, Newcastle, the Gold Coast and Perth. Even an unwashed inland Colorbond fence will collect enough dust in a year to start chalking the surface — and chalking is the precursor to actual paint failure.

Roof cleaning — the safety story

If you’re cleaning a Colorbond roof, the safety considerations matter more than the cleaning method. Wet metal roofing is genuinely slippery — soap on top of polymer is treacherous. Hire a roof harness anchor, a temporary anchor, and a fall-arrest harness from Kennards if you don’t already own one. Or — and this is what I do for any roof over single-storey — 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. Two hospital stays would cost more than 20 years of professional cleans.

If you’re going to do it yourself: never alone, never wet, ladder tied off, harness anchored, and only on the perimeter strips you can reach safely. The middle of a single-storey roof might be reachable from a long-handled extension pole on a roof ladder; the middle of a double-storey roof is not a DIY job, full stop.

What about moss and lichen on the roof?

South-facing Colorbond roofs in shaded gardens grow moss and lichen, especially in temperate Sydney/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 sprayer) applied on a dry day. Leaves it on, doesn’t rinse off — and over the next 6-12 weeks of weather cycles the moss/lichen dies and washes off naturally with rain. It’s the slowest method but the only one that doesn’t damage the surface. Pressure-wash moss off Colorbond and you take the polymer with it.

Bird droppings — act fast

Bird droppings are mildly acidic and if left on Colorbond for weeks they can etch the polymer. The fix isn’t a special product — just rinse them off with the hose within a day or two of noticing. If they’ve already dried and won’t rinse, soak with a microfibre cloth wrung out in the soap solution from step 3, leave 5 minutes, then wipe and rinse. Don’t scrape with a metal blade.

The Priya rule

Soft broom, mild detergent, gentle pressure, hose rinse — that’s it. The pressure washer stays in the shed for the concrete 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 from the US is wrong for Aussie Colorbond — different chemistry, different rules.

Got a stain on Colorbond you can’t shift, or an unusual situation (paint overspray, fire smoke residue)? Send us a write-up.