Rust marks on a Colorbond fence: usually not the fence rusting
Most rust on a Colorbond fence is not the sheet failing. It is transfer staining — swarf from drilling, a steel tool left leaning, run-off from an unpainted fixing — sitting on top of the coating, and it washes off. BlueScope's TB-4 asks for regular fresh-water washing, more often near the coast. Once the coating is breached and the steel edge has gone, cleaning is cosmetic.
Most of it is somebody else's steel
Drill or grind steel anywhere near a Colorbond fence and you throw swarf — hot metal filings that land on the coating and stick. Installing a gate does it. So does a tradesman cutting brackets over the driveway, or an angle grinder two doors down on a still day. The filings are bare steel. The first dew rusts them, the next shower washes the orange downward, and you get fine speckles with little tails running down a panel that was clean last month.
The rest of the usual suspects lean on the fence rather than land on it. A steel garden fork, a wheelbarrow tipped against a panel, a mower parked in the same spot all winter, a plant stake wired to the rail. Each leaves its own shape in orange. So does an unpainted or wrong-grade screw, bracket or hinge, which weeps a neat vertical streak beneath itself every time it rains. None of that is the fence corroding. It is other metal staining the fence.
Read the pattern before you price a new fence
The pattern tells you nearly everything. A fine orange speckle scattered across one or two panels, heavier on the side facing where work happened, is swarf. A single clean streak running down from a fixing point is that fixing, not the sheet. A crescent, a circle or a straight bar of orange is a contact mark from whatever was leaning there. In all three cases the colour sits on the surface, the coating underneath is unbroken, and a damp cloth moves some of it before you have even started properly.
Now the pattern that is not staining. Look along the bottom rail and at any cut edge — the base of the sheets, trimmed post caps, a gate cut down to fit. Bubbling or lifting paint, a brown lacy line along the bottom, or rust you can feel as roughness rather than see as film means the coating has been breached and the steel underneath is going. Soil, mulch and sprinklers piled against the bottom rail are the usual cause. That one is not a cleaning problem.
BlueScope's own instruction is a hose
The manufacturer is not mysterious about this. BlueScope's TB-4 bulletin requires regular fresh-water washing of coated steel, more often near marine environments, and warns against harsh treatment. That is the whole maintenance regime for Colorbond: rinse the salt and grime off before it has time to work, and do not attack it. Rain does part of the job for you on exposed runs, which is why marks tend to appear first where rain never reaches.
Those sheltered spots are worth naming, because they are where the trouble starts. Under an eave or a pergola. The garden side of a fence hidden behind shrubs. Behind the bins. Below a carport line. Grime and salt sit there permanently, and swarf that lands there never gets rinsed off. Washing a Colorbond fence means washing the parts nobody looks at, both sides where you can reach, not just the run the street sees.
Salt turns washing into a schedule
TB-4's line about marine environments is the operative one for half of Sydney. Airborne salt drifts inland from the water, lands on the fence and stays there, holding moisture against the coating long after everything looks dry. If you are in Vaucluse, Bronte, Manly or on Middle Harbour, your fence collects more of it than the same fence in Wahroonga, and it collects it year-round. Same product, same warranty, harder posting. The bulletin's answer is not a different fence — it is washing more often.
Salt also decides how fast a breach turns into a hole. A cut edge on a fence three streets from the beach has bare steel exposed to permanently damp salt, and it goes quickly. Washing does not stop that; it removes the salt before it does its work, which is a real difference over years and no difference at all in a week. On coastal jobs we are checking the bottom rail and the cut edges as much as the flat faces.
What a wash fixes, and what it will not pretend to
A wash lifts transfer staining, road film, salt, mould and cobwebs, and it usually returns the panel to an even colour that surprises people. It does not repair a breached coating. If the paint has blistered, if the bottom edge has gone lacy, if there is a perforation you can push a screwdriver through, cleaning is cosmetic — the panel will look better and still be on its way out. We will say that on site rather than wash it and let you find out next winter. Replacing a panel is a fencer's job, not ours.
Fence work is quoted per job, because length, height, material, how much growth is against it and how tight the access is all move the number, and a flat rate would just be a guess dressed up. The instant quote is not a guess: answer a few questions and it returns a firm, bookable price in about 60 seconds, and that price holds unless the place is materially different from what you described. Add the house wash or the driveway to the same visit and the bundle saves 5-15 per cent.