Painting over mould is the most common, and most expensive, mistake homeowners in the Illawarra make when dealing with a mould problem. The discolouration disappears for a few weeks, then bleeds through the new paint, often worse than before. Proper surface preparation before repainting is not about choosing the right tin of paint, it is about ensuring the substrate is clean, dry, and treated so paint has something sound to bond to. This guide covers every step, from professional remediation through to the final topcoat. For review of specific paint and primer products, see our anti-mould paint and primers review.
Quick Answer (BLUF)
Never paint over active mould. The mould must be treated and the surface must be dry, clean, and primed with an appropriate stain-blocking and antimicrobial primer before any topcoat goes on. In Illawarra conditions, skipping any of these steps results in mould returning through the paint within 3-6 months.
Step 1, Fix the Moisture Source First
Surface preparation means nothing if the moisture source driving the mould is still active. Before any cleaning or painting:
- Confirm the source of moisture: condensation, leak, rising damp, or plumbing fault
- Repair the source completely, have plumbing leaks fixed by a licensed plumber, roof leaks by a licensed roofer, and drainage issues by a contractor
- Allow the affected area to dry completely, use a dehumidifier and fan if needed. In an Illawarra summer, walls can take 3-7 days to dry to acceptable painting moisture levels; in winter they may take longer
Wollongong’s climate adds complexity here. Even after a repair, ambient humidity may keep wall surfaces above the acceptable moisture content for painting (typically <16% for plasterboard). A moisture meter reading is essential before you prime.
Step 2, Assess What Can Be Saved vs What Must Be Replaced
Not every mould-affected wall can be painted over, some require substrate replacement first:
Can be treated and repainted:
- Mould on painted plaster or plasterboard surface where the board is still firm and dry
- Light to moderate mould staining on timber skirtings and architraves
- Mould on rendered masonry (brick, blockwork, concrete) where the render is sound
Must be replaced before repainting:
- Plasterboard that is soft, spongy, crumbling, or has been wet for an extended period, see our plasterboard replacement guide
- Render that is hollow, delaminating, or has rising salt efflorescence
- Timber elements that are structurally compromised
Step 3, Antifungal Treatment
Before any primer, treat the mould properly, do not simply wipe and paint over it.
Professional treatment via a mould remediation service or a professional antifungal fogging treatment is the most reliable option, particularly for large areas or areas with repeated recurrence.
For small areas, DIY treatment process:
- Wear P2 respirator, nitrile gloves, and eye protection
- HEPA vacuum the surface to remove loose spores before wetting
- Apply an antifungal solution, quaternary ammonium-based or hydrogen peroxide-based products are most effective on painted surfaces. Bleach-based solutions work but break down organic material in the substrate over time with repeated use.
- Allow full dwell time (typically 10-15 minutes) before wiping
- Allow the surface to dry completely before the next step, 24-48 hours minimum in the Illawarra
Step 4, Prepare the Surface for Primer
Once treated and dry:
- Sand lightly, P180, P220 grit to remove any raised or blistered paint from the mould-affected area. Wet-sand if airborne dust is a concern.
- Fill any cracks or imperfections, mould often grows in hairline cracks where moisture accumulates. Fill with a flexible acrylic filler, not rigid plaster, in areas prone to movement.
- Wipe down with a tack cloth, remove sanding dust before priming.
- Check moisture readings again, if the wall has been wet during cleaning, confirm it has dried back down before applying any primer.
Step 5, Choose the Right Primer
The primer is the most important product in your system, it is what stops stain bleed-through and provides adhesion for the topcoat.
For mould-affected surfaces, use a stain-blocking alkyd or shellac-based primer:
- Shellac-based primers (e.g., Zinsser BIN) are the most reliable stain-blockers and are excellent at sealing smoke, water stains, and mould pigment. They are shellac and alcohol-based, so ventilate heavily during application.
- Alkyd-based stain-block primers (e.g., Zinsser Bulls Eye 1-2-3 Plus) offer stain blocking with a water-clean-up formula and include a mould inhibitor in the formulation.
- Avoid standard PVA or acrylic undercoats, these will not seal mould staining and may not adhere well to a contaminated surface.
Apply the primer generously over the treated area, extending 10-15 cm beyond the visible staining to catch any spore migration.
Step 6, Topcoat Selection
Once the primer is fully cured (check the manufacturer’s recoat time, typically 1-4 hours for shellac, 2-4 hours for water-based):
- Use a low-sheen or semi-gloss acrylic topcoat, gloss levels make cleaning easier and reduce moisture absorption
- Choose a topcoat with a mould inhibitor (fungicide added), most major Australian paint brands (Dulux, Taubmans, Bristol) have a “mould inhibitor” version of their interior ranges
- In bathrooms, laundries, and other high-humidity spaces, use a bathroom-specific formulation with enhanced mould and moisture resistance
In the Illawarra, ceiling paints in bathrooms without adequate ventilation need reapplying every 3-5 years regardless of quality, plan for this as part of your property maintenance schedule.
FAQs
How long should I wait after mould remediation before painting?
Wait until surface moisture readings are in the normal range, below 14-16% for plasterboard and below 18% for timber. This is typically 3-7 days after treatment in summer conditions with adequate ventilation, and longer in winter or without drying equipment.
Can I use mould-resistant paint without a primer?
No. Mould-resistant topcoats contain a fungicide that inhibits future mould growth, but they will not seal existing staining and will not adhere well to a contaminated surface. The primer is not optional.
Does painting over treated mould put the mould spores back into the air?
If the surface has been properly treated and dried, painting over it does not re-aerosolise spores in meaningful quantities. The risk is mechanical disturbance (sanding), which is why PPE during sanding and vacuum collection of dust are important.
How do I stop mould coming back through the paint?
Moisture control is the only long-term answer, address ventilation, fix the moisture source, and ensure adequate exhaust in wet areas. Paint with a mould inhibitor will slow surface mould growth but will not stop it if the substrate remains damp.