How to Paint Over Mouldy Silicone Around a Window

Mouldy silicone around a window is a common problem in New Zealand homes, especially in humid regions or rooms with poor ventilation. The honest answer is that you cannot simply paint over silicone — standard paints won't bond to it, and painting over mould just traps the problem underneath. The correct fix involves removing the old silicone, treating the mould, re-sealing with a paintable product, and then applying your topcoat.

Why Paint Won't Stick to Silicone

Standard silicone sealant has a non-porous, flexible surface that most paints — including acrylics and oil-based products — cannot grip. Even if you manage to get a coat to sit on top temporarily, it will peel or flake within weeks. On top of that, painting over mould without killing it first means the mould will continue to grow beneath the surface, potentially spreading to the surrounding wall or window frame.

What You'll Need

• Silicone remover or a sharp utility knife and scraper
• Mould treatment solution (e.g. a diluted bleach solution or a dedicated mould killer)
• Clean cloths and a stiff brush
• Paintable acrylic sealant (not standard silicone — check the label says "paintable")
• Painter's masking tape
• Primer suitable for the substrate (timber, plaster, or fibre cement)
• Quality exterior or interior paint, depending on the window location

Step 1 — Remove the Old Silicone

Use a silicone remover product (available at Bunnings or Mitre 10) to soften the old sealant, then carefully cut and peel it away with a utility knife or scraper. Take your time — leaving behind residue will prevent the new sealant from bonding properly. Clean the joint thoroughly once the bulk of the silicone is gone.

Step 2 — Treat the Mould

This step is critical. Mix a solution of one part household bleach to four parts water, or use a dedicated mould treatment product. Apply it to the affected area, leave it to dwell for at least 10–15 minutes, then scrub with a stiff brush and rinse well. Allow the surface to dry completely before moving on.

Safety note: Work in a well-ventilated area when using bleach or mould treatments. Wear gloves and eye protection. If you find extensive mould growth inside the wall cavity or on structural framing, stop work and consult a professional — widespread mould can be a health hazard and may indicate a moisture or weathertightness issue that needs proper investigation. WorkSafe NZ has guidance on working safely with mould.

Step 3 — Re-seal with a Paintable Sealant

This is the key product choice. Do not use standard silicone — instead, choose a paintable acrylic sealant or a hybrid sealant labelled as paintable. These products flex with the building movement (important in NZ given our seismic activity) while accepting a paint topcoat. Apply it neatly using masking tape to get clean lines, tool it smooth with a wet finger, and remove the tape before the sealant skins over. Allow it to cure fully according to the manufacturer's instructions — typically 24 hours minimum before painting.

Step 4 — Prime and Paint

Once the sealant is fully cured, apply a suitable primer to any bare timber, plaster, or fibre cement around the joint. James Hardie substrates, for example, have specific primer requirements — always check the manufacturer's recommendations. Then apply your chosen topcoat. For exterior windows, use a quality exterior paint rated for NZ conditions. Resene, Dulux, and Wattyl all offer exterior waterborne paints well suited to NZ's variable climate — visit their websites or a local store for product advice.

For interior windows in bathrooms or kitchens, choose a mould-resistant paint specifically formulated for wet areas.

Preventing Mould Coming Back

• Improve ventilation — open windows regularly or install an extractor fan. Under the Healthy Homes Standards, rental properties must meet specific ventilation requirements.
• Check that window flashings and exterior seals are intact and weathertight. The NZ Building Code clause E2 (External Moisture) sets out requirements for keeping water out of the building envelope.
• Consider a mould-inhibiting paint additive or a purpose-formulated mould-resistant topcoat in high-humidity areas.
• BRANZ has excellent resources on moisture management in NZ homes.

When to Call a Professional

Call a qualified painter or building professional if:

• The mould keeps returning despite treatment — this may signal a weathertightness defect or condensation problem requiring investigation.
• You find mould inside the wall cavity, on framing, or on insulation.
• The window joinery or surrounding substrate is rotted or damaged — repairs to structural elements may require a Licensed Building Practitioner (LBP). Check lbp.govt.nz to find one.
• You're unsure whether the work requires a building consent — always check with your local council, as some repairs around windows can trigger consent requirements depending on scope.

A professional painter will also have access to commercial-grade mould treatments and sealants that can provide a longer-lasting result. You can find qualified local painters on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ — Moisture & Mould Guidance
• MBIE Building Performance — NZ Building Code
• WorkSafe NZ — Mould Safety
• Resene — Paint & Coating Advice
• Tenancy Services — Healthy Homes Standards
• Licensed Building Practitioners