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Shed & Colorbond
Why Colorbond goes chalky, and what to do about it
Run your hand down an older shed wall and it comes away with white powder on it. That is not dust and it will not wash off with a hose. It is the paint itself breaking down, and how you clean it decides whether the wall comes back even or blotchy.
Get your free quoteWhat chalking is
The powder is the paint, not the dirt
Colorbond is steel with a baked-on paint system over it. Ultraviolet light slowly breaks down the resin that binds that paint together, and as the resin goes, loose pigment is left sitting on the surface. That loose pigment is the chalk on your hand. The colour looks faded because you are no longer looking at the paint film — you are looking at powder lying on top of it.
It is a normal, gradual part of how the coating ages, and up on the Downs it happens faster than most people expect. High UV at elevation, big daily temperature swings, and long dry spells that leave dust and fertiliser residue sitting on the sheet all push it along.
The north and west walls always go first, because they take the most sun. That is why a shed can look almost new from the driveway and tired from the paddock side, and why cleaning only the side you see tends to make the difference more obvious rather than less.
Chalking is different from the other two things people lump in with it: the grey-green film on the shaded southern side is algae and lichen, and the orange speckling near the ground is usually red soil splash-back or steel fixings weeping rust. All three can sit on the same shed and each needs handling differently.
Getting it wrong
Six things that ruin a Colorbond wall
The method
Lift the chalk off evenly, then stop
The whole job is about consistency. A mild alkaline detergent is applied at low pressure across the full elevation, given time to lift the loose pigment, then agitated with a soft brush where it needs it and rinsed from the bottom up so nothing dries part-cleaned. Every wall gets the same treatment whether it looks like it needs it or not.
Where there is algae on the shaded side as well, that gets a separate treatment that kills it at the root rather than scrubbing it off, because scrubbing a chalked sheet is exactly what creates the bright patches. Rust weeping from fixings is a different problem again and is worth flagging to whoever maintains the shed, not chemically attacked.
Water goes on at roughly garden-hose pressure, kept off the laps and screw lines, and the concrete apron and any paths get washed down afterwards so the run-off is not left to dry into the slab. If the shed is due for recoating, a proper clean is the necessary first step anyway — paint will not hold on a chalked surface.
Common questions
Chalky Colorbond, answered
Will cleaning bring the colour back?
It brings back an even finish and a surprising amount of depth, because you are removing a layer of pale powder that has been muting the colour. It cannot restore resin that UV has already broken down, so a badly chalked wall will still read as older than new sheeting — just uniform instead of patchy.
Does chalking mean the shed is failing?
No. It is normal ageing of the coating and the steel underneath is usually fine. What matters is what is happening at the fixings, the laps and the bottom edge, because that is where water sits and where rust actually starts. A wash is a good opportunity to have a proper look at those.
Can I just pressure wash it myself?
You can, and the two things that usually go wrong are pressure into the sheet laps and uneven coverage. Keep the pressure low, keep the lance square to the sheet rather than angled up under it, work the whole elevation at once, and rinse from the bottom so nothing dries halfway through.
How often should a shed be washed?
Every year or two keeps chalk and grime from building to the point where the difference between walls becomes obvious. Sheds near cropping country, unsealed roads or stock yards collect far more and are worth doing annually. Rain only ever washes the parts that are exposed, which is what creates the streaked look.
What about the roof of the shed?
Shed roofs chalk too, and they matter more if the shed feeds a rainwater tank. That is height work and gets done under the same Working at Heights training and signed SWMS as any other roof, with the tank inlet isolated before anything starts.
Do you clean inside the shed as well?
We can, and it is worth asking about if the shed is being cleared out or prepared for sale. Internal walls, the underside of the roof sheets and the slab are all doable, but they need planning around what is stored in there, so it is quoted separately from the exterior.
Next step
Want to know what your shed will come up like?
Send photos of the worst wall and the best one. We will tell you honestly how much of the difference is chalk and how much is age. Free quote, no obligation.
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