How to Remove Mould from Grout Permanently
Two summers ago I cleaned the same Erskineville bathroom three times in six months. Each time the black mould in the grout came back within ten weeks. The owner was furious — at me, at her landlord, at the universe. Right, gear first: I sat her down and explained that bleach decolours mould, it doesn’t kill it. The roots are still alive in the porous grout, and they regrow as soon as the bathroom warms up after a shower. We swapped to the hydrogen peroxide + cling film method I’m about to describe, and 18 months later her grout is still white. The science matters here — if you don’t understand why the standard methods fail, you’ll keep redoing the same job forever.
Gear you’ll need
- Hydrogen peroxide 3% — pharmacy aisle, about $4 a bottle
- Bicarb soda — Woolies Essentials, $1.20
- A small bowl and a soft toothbrush
- An old credit card or plastic scraper
- Cling film (Glad or Coles brand)
- Cleaning gloves and a P2 mask (Bunnings, $4) — you’re disturbing live spores
- Optional: a tube of mould-resistant grout sealer (Davco or Selleys, $18)
- A dry microfibre cloth and a damp one
Step 1: Ventilate first — this is non-negotiable
Open windows, run the exhaust fan, leave the door open. You’re about to release mould spores into the air when you scrub — breathing in a sealed bathroom full of disturbed spores can trigger asthma, sinus issues and persistent coughs. Mask on. If your bathroom has no window and only a weak fan, do this on a sunny day with the rest of the house open and a pedestal fan blowing air out through the doorway.
Step 2: Mix a peroxide and bicarb paste
One part hydrogen peroxide to two parts bicarb soda in a small bowl. Stir to a thick toothpaste consistency. Here’s the science: hydrogen peroxide is an oxidiser that genuinely kills mould cells by disrupting the cell wall, unlike chlorine bleach which mostly decolourises the pigment and leaves the live hyphae intact in the grout pores. The bicarb is the carrier — it holds the peroxide in place on a vertical surface long enough for it to penetrate. Without the carrier, the peroxide runs off in 20 seconds and does nothing.
Step 3: Apply paste thickly to every mouldy grout line
Use the toothbrush to scoop and press paste right into the grout — a continuous bead, not a thin smear. Cover every visible mould spot plus 2cm either side, because the mycelium (the root network) extends further than the visible discolouration. If you only treat what you can see, the spores 2cm away will repopulate within weeks. Press the paste in firmly so it’s filling the grout pore, not just sitting on top.
Step 4: Cover with cling film and walk away
This is the step nobody on YouTube does, and it’s the reason their methods fail. Press cling film over the paste so it stays damp and stuck to the surface. The peroxide needs four to six hours of contact time to penetrate fully into the grout pores. If the paste dries out at 30 minutes, you’ve only killed the surface mould and the deep spores are untouched — it’ll be back in a fortnight. Cling film locks the moisture in. Dwell time is everything.
Step 5: Set a timer — leave for at least four hours, overnight is better
Go to work, watch a movie, do something else. The chemistry needs time. I usually start this job at 7pm and peel the film off at 8am the next day — and that’s overkill in a good way. The peroxide breaks down into water and oxygen as it reacts, so it’s not corrosive to grout even after 12 hours of contact. Definately don’t try to rush this step — short-cycling kills the entire result.
Step 6: Peel off the cling film and scrub
The paste should still be damp. Scrub firmly along each grout line with the soft toothbrush. The mould should lift cleanly — you’ll see black or pink streaks coming off into the paste residue. If a section is stubborn, re-apply paste to that spot and give it another two hours under fresh film. Don’t move on with stubborn patches still showing — those are the spores you didn’t reach, and they’ll regrow first.
Step 7: Rinse and dry thoroughly
Wet microfibre to wipe the paste residue off, then a dry microfibre to fully dry the grout. Wet grout regrows mould fast — drying matters more than you’d think. If you live in a humid Sydney summer, open the window and run the fan for 30 minutes after drying to pull residual moisture out of the porous grout.
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 deep in the cracks where peroxide can’t reach. Run your fingernail along the grout — if it crumbles or you see dust come away, the grout is structurally gone. Skip to step 10. If the grout is solid and just clean now, go to step 9.
Step 9: Apply a mould-resistant grout sealer
Wait until the grout is fully dry — 24 hours after the clean. Paint a clear mould-resistant grout sealer (Davco Colourseal Plus, or Selleys’ equivalent) along the grout lines with a small brush. Two thin coats, an hour apart. The sealer sits in the porous grout and creates a hydrophobic layer that stops spores settling and roots from anchoring. Re-apply yearly in wet areas — it wears off slowly. Mask the surrounding tiles with painter’s tape if you’re worried about runs, although most modern sealers wipe off tile cleanly.
Step 10: When to regrout instead
If the grout is broken, eroded, or if mould keeps coming back within months despite proper cleaning and sealing, the grout is structurally compromised and no chemistry will save it. Time to remove and replace. An oscillating multitool with a grout removal blade gets the old grout out, then re-grout with a mould-inhibiting epoxy grout (Davco Sanitised or Mapei Kerapoxy). It’s a half-day job and worth doing properly — see our regrouting tutorial for the full process. While you’re at it, check the silicone seal at the floor-wall junction; if that’s mouldy through, our re-silicone tutorial covers the proper cut-out-and-replace method.
When you should NOT DIY this
If mould is spreading across plasterboard ceilings, behind tiles, or out from under the skirting onto adjacent walls, that’s no longer a grout problem — that’s a waterproofing failure under AS 3740 and a professional mould remediation job. Widespread mould on porous surfaces (gyprock, MDF cabinetry, behind painted walls) means the spores are colonising the substrate itself, and DIY peroxide on the surface doesn’t fix the source. Same applies if anyone in the household has asthma or immune issues — disturbing established mould releases millions of airborne spores, and that’s a real health risk worth paying a remediator $400-800 to handle properly.
Common screw-ups
- Using bleach instead of peroxide — decolours the mould but doesn’t kill the roots, comes back within weeks.
- Skipping the cling film — paste dries in 30 minutes and you’ve only treated the surface layer.
- Treating only the visible mould — the mycelium extends 2cm further than what you can see, always go wider.
- Not drying after rinsing — wet porous grout reinoculates faster than dry grout ever does.
- Sealing damp grout — traps moisture under the sealer and you’ve built a mould incubator.
Why bleach gets sold for this and why it shouldn’t
Walk down the cleaning aisle at Coles or Woolies and 80% of the “mould remover” sprays are sodium hypochlorite (bleach) — Exit Mould, White King Mould Remover, the supermarket equivalents. They work in the demo: spray, watch the black mould go white in 60 seconds, walk away thinking it’s solved. It isn’t. Bleach is a great chromophore destroyer (which is what made the colour shift) but it doesn’t penetrate porous grout to where the hyphae live. The fungus regrows clear and uncoloured for a few weeks, then darkens again as new spores mature. You’re stuck on a four-week cycle of “spray and watch the mould reappear”. Peroxide costs $4 a bottle from the chemist, actually kills the cells, and the cling-film step holds it there long enough to penetrate. Skip the fancy spray, just use peroxide.
Pink mould vs black mould — same method, different urgency
Pink “mould” in the shower is actually Serratia marcescens, a bacterium that thrives on soap residue and warm moisture. It’s mostly harmless cosmetically but it indicates a humidity problem worth fixing. Black mould in grout is usually Aspergillus or Cladosporium, and at scale it’s a respiratory health issue (especially for asthmatics and small kids). Both respond to the peroxide-and-cling-film method but the pink stuff usually only needs 2 hours of dwell while the black needs the full 6+. Same kit, same chemistry — just adjust the timer.
Cost & time
Materials: about $12 for peroxide, bicarb, cling film and mask if you don’t own them. Sealer is another $18 if you’re sealing afterward (recommended). Time: 15 minutes of hands-on work for paste application and scrubbing, plus 4-12 hours of unattended dwell time. Realistically a whole-bathroom mould treatment is a half-day job — start it Saturday morning, finish Saturday evening.
Skip the fancy spray, just use peroxide and bicarb under cling film. Bleach is the most marketed mould product on the shelf and it’s also the worst-performing one for actually killing the organism. Peroxide plus dwell time plus the cling film seal is the only method I’ve used that gives results that last more than a season. If the mould comes back within months despite doing all this properly, the issue isn’t the cleaning — it’s the moisture source. Check the exhaust fan flow rate, check for leaks behind the silicone, and look at how long the bathroom stays humid after a shower. Mould is a humidity problem first and a cleaning problem second. Caddy out, let’s go.
