How to Re-Silicone a Bathroom (Bath, Shower, Basin)
By Steve — flooring and wet-areas specialist, Adelaide Hills SA, 20 years on the job.
Every six months I get the same phone call. “Steve, I re-siliconed the bath last year and it’s already gone black again. Is it a dud tube?” It’s almost never the silicone. Selleys, Sika, Bostik — all of them work fine if you apply them correctly. The reason your bead’s gone mouldy in six months is that you didn’t kill the spores in the substrate before you sealed over them. You painted a fresh coat over a contaminated surface and trapped the mould inside, where it’s now happily fed by trapped moisture and sealed away from any cleaner you might use.
The fix is a proper kill step — not a wipe with bleach, not a spray of mould remover. A genuine soak with a diluted bleach solution, full dwell time, then a complete dry-out, then re-silicone. Skip that step and the new bead is dead the day you apply it.
The other thing nobody mentions in the bunnings-aisle videos: there are two kinds of silicone for bathrooms and only one of them is actually rated for permanently submerged joints. AS 3740 (waterproofing of domestic wet areas) is the standard your bathroom should have been built to, and the silicone you re-do it with should be compliant or you’re voiding the original waterproofing. I’ll name names below.
What you’ll need
- Sharp Stanley knife with new blade
- Silicone scraper tool (Selleys or Wallboard Tools — about $7) OR a plastic putty knife
- Methylated spirits
- White King bleach (or any 4% sodium hypochlorite) — diluted 1:4 with water
- Spray bottle
- Paper towel and microfibre cloths
- Hair dryer or fan heater
- Selleys Wet Area 3-in-1 (rated for AS 3740 submerged joints) — NOT Selleys 401 for bath/shower joints
- Caulking gun (Bostik or Sika brand, not the cheap $4 ones — they don’t release pressure cleanly)
- Masking tape (3M or Norton 36 mm low-tack)
- Smooth tooling tool (Cap Caulk, or just a clean wet finger if you trust yourself)
- Nitrile gloves and safety glasses
Step 1: Cut out 100% of the old silicone
This is non-negotiable. Any old silicone left behind contaminates the new bead — silicone doesn’t bond to silicone reliably. Score along both edges with the Stanley knife, getting the blade right into the corner. Then run the scraper through the joint to lift the bead out. Pull the long ribbons of silicone off — they should come away in continuous strips. Anywhere that doesn’t, score and scrape again.
Step 2: Methylated spirits wipe to remove residue
Even after scraping, there’s an invisible silicone film left on the surface. Wipe the entire joint area with metho on a paper towel, then dry. This dissolves the residue and removes any soap scum or body oil that would prevent adhesion.
Step 3: The mould kill — this is the step everyone skips
Mix 1 part White King bleach to 4 parts water in your spray bottle. Spray the entire joint generously and let it sit for 30 minutes. Re-spray if it dries before 30 minutes is up. The dwell time matters — bleach kills mould by oxidation and that’s a slow chemical reaction. A quick wipe doesn’t do it. After 30 minutes, rinse with clean water and wipe dry. If you still see grey shadows in the grout or substrate, repeat — the spores are still alive.
Step 4: Dry the joint completely — overnight minimum
Silicone won’t cure properly against a damp substrate, and trapped moisture is exactly what feeds new mould. Run a fan heater or hair dryer along the joint for 20 minutes, then leave the bathroom unused with the extractor running overnight. The joint must be bone dry before you cap it. If you re-silicone over residual moisture, you’re sealing in the conditions for mould to come right back.
Step 5: Choose the right tube — Selleys Wet Area 3-in-1
Here’s the gotcha. Selleys 401 is a popular general-purpose silicone, but it’s not rated for permanently submerged joints. For bath, shower base and basin joints — anywhere water sits or repeatedly wets the bead — use Selleys Wet Area 3-in-1, which is AS 4858 / AS 3740 compliant. Sika Sanisil and Bostik Wet Area are equivalent alternatives. Using the wrong tube doesn’t just shorten the bead’s life; if you’ve still got a building warranty active, it can void it because the wet-area waterproofing membrane system specifies AS 3740 compliance throughout.
Step 6: Mask the joint precisely
Run a strip of 36 mm masking tape along each side of the joint, leaving a gap of 4–6 mm (the width of your finished bead). Press the tape edge down hard with a fingernail — any lift here means silicone bleeds under and you get a fuzzy edge. The masking tape is what gives you the perfectly straight bead that looks professional.
Step 7: Cut the nozzle to the right size
Cut the nozzle at 45 degrees, with the opening matching your masked gap (4–6 mm). Pierce the inner foil with a long nail or the spike on the caulking gun. Don’t cut the nozzle huge — too much silicone is the most common amateur mistake; you can always do another pass, but you can’t tidy up an over-thick bead.
Step 8: Run the bead in one continuous pass
Steady, even pressure on the trigger. Move at a consistent speed — about 100 mm per second is a good starting pace. Push the gun (don’t pull it) so you’re forcing silicone into the joint, not just laying it on the surface. Push along the full length without stopping if you can — pause-and-restart marks show in the finished bead.
Step 9: Tool the bead immediately
Within five minutes of laying the bead, smooth it with a Cap Caulk tool or your gloved fingertip dipped in a soapy water solution (one drop of dish soap in 200 ml water — too much soap weakens the bond). One smooth pass, full length, applying gentle pressure. The tooled bead should be slightly concave — that’s the strongest joint shape.
Step 10: Pull the masking tape while the silicone is wet
Pull each tape strip away at 45 degrees while the silicone is still tacky. Pulling after it skins gives you a torn edge; pulling while wet leaves the silicone with a clean line. Don’t touch the bead for 24 hours and don’t run the shower for at least 48 hours — Selleys Wet Area cures fully in 7 days but is shower-ready at 48 hours. Read the tube — different products have different cure times.
Common things that go wrong
From two decades’ worth of bathroom rebuilds and remediation:
- Bead skins over but stays gummy underneath. The silicone tube is past its use-by date. Check the bottom of the tube — there’s a date code, and once silicone is a year past it, full cure becomes unreliable. New tube fixes it.
- Black spots return within a month. Mould kill step was incomplete (likely a visible-mould-only kill, not a full 30-minute soak in 1:4 bleach). Cut out the affected section, full kill, full dry, redo.
- Bead pulls away from one side. The substrate moves under load — typical with acrylic baths that flex when filled. Fix is a bath-support strip or building up the bath base before re-silicone, then use a “high modulus” sanitary silicone that handles movement.
- Silicone won’t release from the masking tape and tears. You waited too long — tape needs to come off within 5 minutes of tooling. If the bead has skinned, tape removal pulls the bead out of the joint. Cut along the tape edge with a fresh Stanley blade before pulling.
- White silicone yellows in 6 months. Either the wrong product (paintable acrylic looks like silicone but yellows fast) or repeated exposure to chlorine bleach in cleaning products. Use neutral-cure silicone in spaces cleaned with bleach.
- Joint cracks at the corner. Three-sided joints (where bath meets wall meets wall) are stress points. Tool the corner extra carefully and don’t skimp on bead depth at the corner.
The Steve rule
The mould kill step is the single thing that separates a 6-month bead from a 5-year bead. Bleach soak, full 30-minute dwell, complete dry-out before you re-silicone. Skip it and you’re sealing in living spores. And use AS 3740-compliant silicone for submerged joints — it’s the same price and it’s the legally correct product under the Plumbing Code.
Got a wet area joint that keeps failing despite a proper job? Send us a write-up.
