Mould on Your Walls? How to Remove It Properly Before You Paint

Mould on Your Walls? How to Remove It Properly Before You Paint

Mould on interior walls is one of the most common problems Sydney homeowners face — our humidity, coastal moisture and plenty of older, poorly ventilated homes make ideal conditions for it. And the single most common mistake is understandable but costly: painting straight over it.

Here's why that fails, and how to deal with wall mould properly so it doesn't come straight back.

Why you can't just paint over mould

Mould is a living organism. Painting over it doesn't kill it — it just buries it under a coat of paint that it will happily grow through, usually reappearing as dark spots within weeks. Worse, the moisture feeding the mould stays trapped behind the new paint, which can cause the paint to bubble and peel on top of the mould problem.

Painting over mould is like mowing weeds instead of pulling them. It looks solved for a moment, then it's back.

First, understand the real problem

Mould doesn't appear randomly. It grows wherever there's moisture, poor airflow and something to feed on — and interior paint and plaster are food. So before you treat a single spot, it's worth identifying why it's there:

• Condensation — the most common cause. Warm, moist air (from showers, cooking, drying clothes indoors) hits a cold surface and forms water. Classic in bathrooms, on window reveals, and on the inside of cold external walls.
• Poor ventilation — no exhaust fan, windows kept shut, air that never moves.
• Water ingress — a leak, rising damp, or rain penetration through a defect in the exterior (this overlaps directly with what causes exterior paint to peel).
• Cold spots / thermal bridging — parts of a wall that stay colder and attract condensation.

If you treat the mould but not the cause, you'll be re-treating it every year.

How to remove mould before painting: step by step

1. Fix the moisture source first. This is non-negotiable and comes before anything else. Repair leaks, improve drainage, and address ventilation (more on that below). Everything after this step is wasted effort if the moisture keeps coming.

2. Protect yourself and the area. Ventilate the room, and use gloves and eye protection — and a mask for anything more than a small patch. Mould spores shouldn't be breathed in, and disturbing mould releases them.

3. Treat and remove the mould. Clean the affected area with a suitable mould treatment made for the job, following the product's instructions and dwell time so it actually kills the mould rather than just wiping the surface. Remove the dead mould and residue. Avoid aggressive scrubbing that spreads spores or gouges the surface.

4. Let the wall dry completely. Fully. Painting a wall that's still holding moisture reintroduces the whole problem. Give it time, and use airflow or a dehumidifier to help.

5. Sand and prepare the surface. Lightly sand to remove any staining and residue and to key the surface. Deal with any damaged plaster.

6. Prime with a mould-resistant sealer. A stain-blocking, mould-resistant primer seals the surface, blocks any lingering staining from bleeding through, and gives the topcoat a sound base.

7. Finish with an anti-mould / mould-resistant paint. In wet and mould-prone areas — bathrooms, laundries, kitchens — a quality mould-resistant paint adds an extra layer of defence. But understand its limits: it's a help, not a substitute for fixing the moisture and ventilation.

The step everyone skips: ventilation

Here's the long-term truth. You can treat mould perfectly and use the best anti-mould paint on the market, and it will still come back if the room stays warm, damp and still. Ventilation is the actual cure.

Practical fixes that make the biggest difference:

• Working exhaust fans in bathrooms and laundries, used during and after showers/washing, vented outside (not into the roof cavity).
• Airflow — open windows regularly, don't block vents, leave gaps behind furniture on cold external walls.
• Manage indoor moisture — cover pots when cooking, avoid drying clothes indoors where possible, wipe down shower walls.
• A dehumidifier in persistently damp rooms.

Address the air, and you turn a recurring problem into a one-time fix.

Bathroom and wet-area walls

Bathrooms are Sydney's number-one mould hotspot — constant moisture, often limited ventilation. For these:

• Prioritise ventilation above everything (a good exhaust fan is worth more than any paint).
• Use a proper mould-resistant paint rated for wet areas, in a washable finish (low-sheen or semi-gloss wipes down more easily than flat).
• Keep on top of surface moisture — a squeegee on shower walls genuinely reduces mould load.

When to call a professional

Small patches of surface mould in a well-ventilated room are a reasonable DIY job with the steps above. Call a professional when:

• The mould is widespread, keeps coming back, or covers a large area.
• It's linked to a leak, damp or water damage you can't locate or fix.
• The wall is soft, stained through, or the plaster is damaged.
• Anyone in the home has respiratory issues, allergies or a compromised immune system — significant mould is a genuine health concern, not just a cosmetic one, and may warrant assessment beyond a repaint.
• You want it fixed once, properly — cause diagnosed, treated, sealed and finished so it stays gone.

A painter experienced with mould-prone homes doesn't just repaint the wall — they identify why the mould is there, treat it correctly, and use the right primer and paint system so you're not looking at the same spots next winter.

Dealing with mould on your walls?

At Mertico, we treat and repaint mould-affected interiors across Sydney and the Illawarra — finding the cause, treating the mould properly, sealing with mould-resistant systems, and advising on the ventilation that keeps it gone. We'll assess it in person and give you honest, itemised advice