Why Bathroom Mould Comes Back After Painting, and How to Stop It

If black spots are creeping back across a bathroom ceiling a few months after it was painted, the paint is rarely the real problem. Mould comes back when the moisture that feeds it is still there, or when the old growth was painted over instead of being properly treated. Fix the moisture, treat the surface, then use a coating made for wet rooms, in that order, and a repaint can stay clean for a long time.

This guide explains why mould returns, what a lasting fix looks like, and which jobs are better left to a professional. It is general information, not advice on your own property.

Why mould keeps coming back

Mould is a fungus that grows wherever moisture builds up. NSW Health lists walls, ceilings and bathroom tiles among the common places it appears, with leaks, rising damp and indoor humidity as typical sources. A bathroom has all of that in one small room: hot showers, little airflow and surfaces that stay damp for hours.

When mould returns after a repaint, it usually comes down to one or more of these:

• The moisture source was never fixed. If steam has nowhere to go, or water is getting in from a leak, new spores will settle and grow on fresh paint just as they did on the old.
• Paint went straight over live mould. Paint can hide the stain for a while, but it does not remove the growth underneath. It often shows through again as grey or black patches, sometimes with bubbling or peeling.
• The surface wasn’t properly cleaned and dried. Residue, soap film and damp plaster stop new paint bonding well, and poorly bonded paint traps moisture.
• The wrong paint was used. A flat ceiling paint meant for living rooms is not made for a room that steams up every day.

Start with the moisture, not the paint

Before anyone opens a tin, work out where the moisture is coming from. Condensation from showers is the most common cause in Sydney bathrooms, but it is worth ruling out the others.

• Ventilation. NSW Health recommends running exhaust fans when showering or bathing and opening windows when the weather allows. Check that the fan actually works, that it is clean, and that it vents outside the house rather than into the roof space.
• Leaks. A stain that keeps growing in one spot, especially near the shower, bath or a wall shared with the shower, can point to a plumbing leak or failed waterproofing. That needs a plumber or waterproofer before any painting.
• Rising or lateral damp. Mould low on a wall, with flaking paint or salt deposits, can mean moisture is coming through the masonry. NSW Health suggests an experienced building consultant can check the damp course.

If you rent the property and mould keeps returning despite good ventilation, NSW Health advises raising it with the owner or real estate agent.

How a lasting fix is done

Once the moisture source is dealt with, the surface itself needs proper preparation. On a typical bathroom job, the process looks like this:

1. Protect the room. Fixtures, floors and vanities are covered, and the area is ventilated while work is underway.
2. Treat the mould. Affected areas are cleaned with a suitable mould treatment and wiped down thoroughly. NSW Health recommends gloves and safety glasses when cleaning mould, and good ventilation if bleach is used.
3. Let it dry completely. Rushing this step is one of the main reasons mould reappears.
4. Remove failing paint. Bubbling, flaking or peeling paint is scraped back to a sound edge. Damaged plaster is repaired.
5. Seal with a stain-blocking primer. A primer (the first, bonding coat) suited to wet areas stops old marks bleeding through and gives the top coats a firm base.
6. Apply two top coats of a wet-area paint. Kitchen-and-bathroom paints are made to handle humidity and regular cleaning, and many include a mould inhibitor, an additive that slows mould growth on the paint film. Check the product’s data sheet for its wet-area use and recoat times.
7. Allow proper curing before heavy steam. Paint can feel dry within hours but takes longer to harden fully. The data sheet will say how long to wait before long, hot showers.

Our mould removal service follows this kind of process, so the room is treated and prepared before it is repainted, rather than simply covered up.

Choosing the right finish for a bathroom

Sheen (how shiny the paint is) matters in wet rooms. A low sheen or semi-gloss finish is easier to wipe and resists moisture better than a flat finish, which is why it is the usual choice for bathroom walls. Ceilings in bathrooms also benefit from a paint made for wet areas rather than a standard flat ceiling white. Our paint sheen guide explains how each finish suits different rooms.

Low-VOC paints (paints with fewer volatile organic compounds, the solvents that cause that strong paint smell) are also worth asking about for a small, enclosed room like a bathroom. They are what we use on our jobs.

When to call a professional

Small spots of surface mould can often be cleaned by an owner with care. Some situations are better handed to someone else:

• Large or spreading areas, or mould that returns quickly after cleaning.
• Anyone in the home with asthma, allergies or a weakened immune system. NSW Health notes these people are more at risk from mould exposure.
• Suspected leaks or failed waterproofing, which need a licensed plumber or waterproofer.
• Older bathrooms. SafeWork NSW warns that homes built before the late 1980s may contain asbestos, including in bathroom wall and ceiling sheeting. Never sand, drill or pressure-wash sheeting you are unsure about. Have it checked first, following SafeWork NSW guidance.
• Homes built before 1970, where old paint layers may contain lead. Our guide to lead paint in older homes covers what to look out for.

For strata buildings, mould in a bathroom can be linked to common property such as the roof, external walls or shared plumbing. Check with your strata manager about who is responsible before work starts.

Keeping it clean after the repaint

• Run the exhaust fan during every shower and for a while afterwards.
• Leave the door or window open after showering when you can.
• Clean small spots early with a mild detergent before they spread.
• Get leaks and dripping taps fixed promptly.

Ready for a written quote?

Call 0423 533 883, or tell us about the job and we will reply within one business day. We give a written, itemised quote after a site walk, and our work comes with a five-year workmanship guarantee.

Sources: NSW Health, Mould · SafeWork NSW, Asbestos