Mould on Walls and Ceilings: What You Need to Do Before Painting

Mould on walls and ceilings is one of the most common issues we encounter when inspecting homes across South East Melbourne — and one of the most commonly mishandled.

The short version: painting over mould does not fix it. The new coat hides it briefly, then the mould comes back through the paint faster than it appeared the first time. In many cases, it spreads.

If your walls or ceilings have mould, the correct sequence is treat first, paint second. Here’s what that actually means.

Why Painting Over Mould Makes It Worse

Paint applied over active mould can trap contamination beneath the new film while the moisture problem remains. The mould may continue to grow under the new coat, weaken adhesion, and eventually break back through the surface.

You end up with peeling paint and returning mould. The solution costs more to fix the second time because the paint failure has to be addressed alongside the mould treatment.

The only way to get a paint job that holds up in a mould-affected area is to treat the mould before the primer goes on.

What Kind of Mould Are We Talking About?

The mould most commonly found on interior walls and ceilings in Melbourne homes is surface mould caused by condensation and inadequate ventilation — not structural water damage. It typically appears as:

• Black or grey spots on bathroom ceilings and shower surrounds
• Dark patches in corners, particularly south-facing rooms
• A ring of dark staining around ceiling air vents or exhaust fans
• Widespread grey-green discolouration on walls in poorly ventilated rooms

This type of mould is treatable with proper preparation before painting. It is a different category to mould caused by active roof leaks, rising damp or failed waterproofing — those require the underlying cause to be fixed before any surface treatment will hold.

How to Treat Mould on Walls and Ceilings Properly

Step 1 — Identify the source

Before treating anything, establish whether the mould is caused by condensation and poor ventilation or an active water source. Check:

• Is the mould only in bathrooms, laundries and rooms without exhaust fans or operable windows?
• Is there a roof leak or penetration above the affected ceiling?
• Is there visible moisture or staining tracking down the wall (indicating plumbing or external water entry)?

If the mould is associated with an active water source, fix the water issue first. Surface treatment alone won’t hold.

Step 2 — Treat with a biocide solution

Bleach-based or commercial mould treatment products can help treat surface mould when used according to the product instructions. The treatment should be:

• Applied to the full affected area, not just the visible spots
• Left for the dwell time specified on the product — typically 15–30 minutes
• Wiped down and allowed to dry fully before any primer is applied

Products available at hardware stores that work for this purpose include Selleys Rapid Mould Killer, Peel Away Mould Killer and commercial-grade sodium hypochlorite solutions. For extensive mould coverage, a professional treatment is more reliable than DIY application.

Step 3 — Allow full drying before painting

The treated area must be completely dry before primer is applied. Painting over a damp surface — even a treated one — creates ideal conditions for mould recurrence. In high-humidity rooms, this may require ventilating the room for several days before proceeding.

Step 4 — Apply a stain-blocking primer

Standard primer over a previously mould-affected surface is not sufficient. A stain-blocking primer — such as Zinsser BIN, Dulux Sealer Primer or similar — seals the treated surface and prevents bleed-through of staining into the new paint coat.

Step 5 — Paint with appropriate finish

The topcoat for a previously mould-affected area should be:

• In bathrooms and wet areas: a moisture-resistant paint with a semi-gloss or satin finish (lower sheen finishes trap moisture on the surface)
• In living areas and bedrooms: a standard interior paint — mould-resistant formulations are available and worth considering for rooms with recurring issues

When Is It a Ventilation Problem, Not a Paint Problem?

Mould-resistant paint does not prevent mould in a room with inadequate ventilation. It slows the rate of recurrence — it does not eliminate it.

IIf you have recurring mould on bathroom ceilings after every repaint, ventilation is usually one of the first things to check:

• Exhaust fans are undersized for the room
• Exhaust fans ducting into the roof cavity rather than to the exterior (a common installation defect in SE Melbourne homes)
• No cross-ventilation and no consistent fan use during showers

Addressing the ventilation is the permanent fix. Painting is part of the maintenance cycle — but it is not a substitute for dealing with the moisture source.

Signs the ventilation is the problem:

• Mould returns within 12–18 months of repainting
• Mould is worst on the ceiling directly above the shower
• Condensation visible on windows and mirrors for extended periods after showering

Common Questions

Can I just scrub it off and paint over it?

Scrubbing may remove visible mould from the surface, but it may not treat the underlying contamination and can disturb the affected area. Without suitable treatment and proper drying time, scrubbing followed by painting can still result in mould returning.

Does mould resistant paint work?

Mould resistant paint contains additives that inhibit mould growth on the paint film surface. It does not kill existing mould, and it does not prevent mould in conditions with inadequate ventilation. Used on a properly treated surface in a well-ventilated room, it extends the time before mould returns. It is not a substitute for treatment. We cover this in more detail in our guide to mould resistant paint.

Is it safe to paint over mould myself?

For small areas of surface mould in well-ventilated areas, DIY treatment may be manageable if you follow the product instructions and use appropriate protection. For extensive mould coverage, mould in multiple rooms, or any situation where you suspect the source is a structural water issue, professional assessment is worth the cost.

Will a painter just paint over it without telling me?

Some will. We don’t — and we are upfront about why. Painting over untreated mould produces a result we can’t stand behind. Where we see active mould during an inspection, we scope and price the treatment as part of the job.

What We Do on Mould-Affected Jobs

When we inspect a property and find mould on walls or ceilings, we include the treatment in the preparation scope:

• Full biocide treatment of affected areas
• Drying time factored into the job schedule
• Stain-blocking primer before topcoat
• Correct finish specified for the room type
• Ventilation advice given where the source appears to be condensation

This is included in the written fixed-price quote — not charged as an extra after the job starts.

If you’re looking to repaint a bathroom, laundry, or any room with visible mould, Request a Free Written Quote. We’ll inspect the surface, assess what’s needed and give you a clear scope before any work starts.

Melbourne Renovation Experts provides interior painting and low-tox painting across South East Melbourne. Based in Glen Waverley. No subcontractors. Written fixed-price quotes.