How to Remove Mould from Grout Permanently

By Priya — IDIY’s deep-clean specialist.

Mould in grout is a Sydney summer special. Anywhere there’s humidity above 60% and not enough ventilation, you’ll get black or pink mould working its way into the grout lines. The mistake most people make is treating it like dirt — scrubbing harder, using stronger bleach, hoping it goes away. It doesn’t. Mould has roots that go deeper than the surface, and you have to either kill the spores at depth or replace the grout.

Here’s the method our team uses to permanently remove mould from grout — and how to keep it from coming back.

What you’ll need

A bottle of hydrogen peroxide (3% — pharmacy aisle)
Bicarb soda
A small bowl and a soft toothbrush
An old credit card or plastic scraper
Cling film
Cleaning gloves and a mask (you’ll be releasing spores)
Optional: a tube of mould-resistant grout sealant

Step 1: Ventilate first

Open windows, run the exhaust fan, leave the door open. You’re about to disturb mould spores — you don’t want to be breathing them in a sealed bathroom.

Step 2: Mix a peroxide and bicarb paste

One part hydrogen peroxide to two parts bicarb soda, mixed in a small bowl until it’s a thick paste like toothpaste. This is the magic mix — peroxide actually kills mould (bleach mostly just decolours it), and bicarb scrubs and holds the peroxide on the surface.

Step 3: Apply paste thickly to every mouldy grout line

Use the toothbrush to scoop and press the paste right into the grout lines. Don’t be shy — you want a continuous bead, not a thin smear. Cover every spot of mould plus 2cm either side, because the spores extend further than the visible discolouration.

Step 4: Cover with cling film and walk away

This is the secret step nobody on YouTube does. Press cling film over the paste so it stays wet. The peroxide needs four to six hours of contact time to penetrate down through the grout and kill the spores. If the paste dries out in 30 minutes, you’ve only killed surface mould and it’ll be back in a fortnight.

Step 5: Set a timer — leave for at least 4 hours

Overnight is even better. Go to work, watch a movie, do something else. The chemistry needs time.

Step 6: Peel off the cling film and scrub

The paste should still be damp. Scrub firmly along each grout line with the toothbrush. The mould should lift cleanly — black streaks coming off into the paste. If it’s stubborn anywhere, re-apply paste to that spot and give it another two hours.

Step 7: Rinse and dry thoroughly

Wet microfibre to wipe the paste off, then a dry microfibre to fully dry the grout. Wet grout regrows mould fast — drying matters.

Step 8: Inspect for grout damage

If the mould had been there for years, the grout itself may be cracked, chalky or eroded. Damaged grout will keep growing mould no matter how much you clean it because spores live in the cracks. If you see crumbling grout, you need Step 10. If the grout is solid and just discoloured, skip to Step 9.

Step 9: Apply a mould-resistant grout sealer

Once dry — fully dry, 24 hours after the clean — paint a clear mould-resistant grout sealer along the grout lines with a small brush. Two thin coats, an hour apart. This sits in the porous grout and stops new spores from settling. Re-apply yearly.

Step 10: When to regrout instead

If the grout is broken, eroded, or if mould keeps coming back within months despite cleaning and sealing, the grout is structurally compromised. Time to remove and replace. A grout-removal tool (oscillating multitool with a grout blade) gets the old grout out, then you re-grout with a mould-inhibiting epoxy grout. It’s a half-day job and worth it — cleaner grout for ten years.

The Priya rule

Peroxide plus dwell time plus cling film. The cling film is non-negotiable. Surface scrubs only delay the mould — peroxide-paste-and-cling-film actually kills it.

Got persistent black mould that’s beaten everything? Send a photo — sometimes it’s not the grout, it’s the silicone, and that’s a whole different fix.