Mould on Exterior Walls: Washing and Treating Before You Repaint in Doncaster

Walk around almost any house in Doncaster, Templestowe or Donvale and you will find at least one wall that looks dirtier than the rest. It is usually the south side, or the wall that sits behind a big tree, a tall fence or the neighbour's double storey. Up close it is rarely just dirt. Black speckling on render, a green film on the lower brickwork, grey streaks running down from the eaves: that is mould, mildew and algae, and it matters a great deal if you are about to repaint.

Here is why Manningham homes grow it, and what has to happen to it before any paint goes on.

Why Manningham houses grow mould

Mould and algae need three things: moisture, shade and something to feed on. Much of the City of Manningham supplies all three. The leafy streets of Donvale, Park Orchards and Warrandyte back onto bushland, and even in Doncaster and Templestowe the established gardens, mature gums and tall hedges keep whole elevations in shade for most of the year. Those walls stay damp long after a Melbourne shower, and the dust, pollen and leaf litter that settle on them give growth a foothold.

Some surfaces are worse than others. Eaves are shaded and poorly ventilated, which is why they are often the first place mildew shows. Rough render and bagged brick hold moisture and dirt in their texture. Weatherboards that sit close to garden beds stay damp along the bottom boards. Overflowing gutters and leaking downpipes soak a section of wall over and over, and the growth follows the water stain exactly.

Why you cannot paint over it

It is tempting to think a fresh coat will cover the problem. It will, for a while. Paint applied over live mould does not bond to the wall; it bonds to the growth and the grime sitting on the wall. The growth keeps living underneath, and within a season or two the dark spots bleed back through the new colour, the coating starts to lift and blister in those areas, and you are paying for the preparation that was skipped the first time.

That is why every exterior repaint we quote under our Exterior House Repainting service starts with a proper wash-down.

How exterior mould is treated before painting

First comes a treatment wash. Affected areas are wetted and treated with a suitable mould and algae wash, and it is left to work for the time the product needs. Water alone moves the visible staining but leaves the growth alive in the surface.

Next is the wash-down. The whole house is then washed to remove the dead growth, chalk, cobwebs, traffic film and dirt. Pressure matters here. A high-pressure blast can drive water behind weatherboards, lift loose render, strip soft mortar from old brick and damage timber, so the method is matched to the surface. Render and brick can usually take more than weatherboards, eaves linings and timber trim, which are cleaned more gently.

Then the house dries. Painting onto a wall that is still damp from washing traps moisture behind the new film, which is one of the most reliable ways to get blistering. On a shaded south wall in autumn or winter that drying time can be considerably longer than on a sunny north face, and a careful painter builds it into the program.

Finally comes priming and coating. Once the surface is clean, dry and sound, bare or chalky areas are primed or sealed, and the topcoats go on. Most quality exterior paints already contain a mould inhibitor in the film, and on the worst-affected walls we discuss whether a product with stronger mould resistance is worth specifying.

Asbestos and older eaves

Many 1960s and 1970s homes across Doncaster, Bulleen and Templestowe have flat fibre cement eaves linings, and sheeting of that age may contain asbestos. It can be washed and painted safely, but it must not be high-pressure blasted, sanded or scraped aggressively. We identify the material before we start and wash those areas gently, which is the same approach we describe in our Weatherboard, Eaves and Fascia Repaints service.

Fixing the cause, not just the symptom

Treatment and good paint buy you a clean finish, but if the conditions stay the same, the growth will eventually come back on the same walls. A few things make a real difference. Cut back shrubs and climbers so air can move along the wall and the surface can dry. Clear gutters and fix leaking downpipes so a section of wall is not being soaked every time it rains. Keep garden beds, mulch and paving from building up against the base of the wall. And give shaded walls a gentle wash every year or two, which removes early growth before it gets established.

Our guide on how often you should repaint a house in Melbourne explains why shaded and weather-facing walls usually set the timing for the whole house.

When mould is a sign of something bigger

Most exterior mould is a surface problem. Occasionally it points to moisture coming from somewhere it should not, such as a failed damp course, a leaking roof valley, cracked render letting water into the wall, or rotten boards behind the paint. If growth keeps returning to the same patch, or the paint there is bubbling and the render sounds hollow when tapped, the wall needs repair before it needs paint. We flag these at inspection rather than painting over them, and on rendered homes the repair side is covered by our Render and Masonry Painting service.

Getting it inspected

If your walls are showing mould and the paint is getting tired, the right time to deal with both is together. We will walk the house, tell you which elevations need treatment, check what the eaves are made of and give you a fixed written quote that includes the wash and treatment properly rather than leaving it vague. Get in touch to arrange a free inspection anywhere in Doncaster and the City of Manningham.