How to Re-Silicone a Bathroom (Bath, Shower, Basin)
Every six months I get the same phone call from one of my regulars in Aldgate or Stirling. “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 — they all work fine when you apply them onto a properly prepped substrate. The reason the bead has gone mouldy in six months is that the spores in the substrate were never killed before the new silicone was laid over the top. The mould was painted over, the new bead trapped the moisture against living spores, and the colony bloomed underneath where no spray cleaner can reach it. Twenty years on the tools and I still see this exact mistake every fortnight. The number of leak callbacks I’ve done because someone wiped over visible mould with a wet cloth and thought that was a kill step would buy me a new ute. Here’s the way to do it once, properly.
What you’ll need
- A sharp Stanley knife with a brand-new blade
- A 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
- A spray bottle
- Paper towel and microfibre cloths
- A hair dryer or fan heater
- Selleys Wet Area 3-in-1 (AS 3740 compliant for submerged joints) — NOT Selleys 401 for bath or shower joints
- A caulking gun from Bostik or Sika (not the $4 cheapies — they don’t release pressure cleanly)
- 3M or Norton 36 mm low-tack masking tape
- A Cap Caulk smoothing tool or a clean gloved finger
- 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 reliably to old silicone, and the join becomes a wick straight into the wall cavity. Score along both edges of the bead with the Stanley knife, getting the blade right into the corner of the joint. Then run the scraper through to lift the bead out. The long ribbons should come away in continuous strips. Anywhere they don’t, score and scrape again. Don’t move on to Step 2 until every smear is gone — including the smudge on the tile faces.
Step 2: Methylated spirits wipe to remove the residue film
Even after careful scraping, there’s an invisible silicone film left on the surface that the eye can’t see. 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 otherwise stop the new bead from adhering. Don’t substitute white spirits or turps — they leave their own residue. Metho is the right product because it evaporates clean.
Step 3: The mould kill — 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 the 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 can still see grey shadows in the grout or substrate, repeat the soak — the spores are still alive. For surrounding tile work that needs attention while you’re in there, see how to regrout tile floor and wall.
Step 4: Dry the joint completely — overnight minimum
Silicone won’t cure properly against a damp substrate, and trapped moisture is exactly what feeds the next mould bloom. Run a fan heater or hair dryer along the joint for 20 minutes, then leave the bathroom unused with the extractor fan 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 that brought you here in the first place. Hill’s water down here is hard, so any mineral residue from a recent wipe-down can also slow cure — bone dry is bone dry.
Step 5: Choose the right tube — Selleys Wet Area 3-in-1 or equivalent
Here’s the gotcha most people miss. 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 and 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 still have a builder’s warranty active on the bathroom, it can void it because the wet-area waterproofing membrane system specifies AS 3740 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 or a smoothing tool. Any lift here and silicone bleeds under to give you a fuzzy edge. The masking tape is what produces the perfectly straight bead that looks professional. Don’t skip it thinking you’ll go freehand — even with twenty years on the tools I still tape. Do it once, do it properly.
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 a second pass, but you cannot tidy up an over-thick bead without starting again. The 45-degree cut also lets you control which way silicone flows when you push the gun.
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 rather than just laying it on the surface. Push along the full length without stopping if you can. Pause-and-restart marks definately show in the finished bead and you can’t tool them out. Practice on a scrap of timber before you commit to the bath if it’s your first one.
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 soapy water (one drop of dish soap in 200 ml water — any more weakens the bond). One smooth pass, full length, applying gentle pressure. The tooled bead should be slightly concave — that’s the strongest joint shape because it concentrates film thickness in the middle of the joint where movement is greatest.
Step 10: Pull the masking tape while the silicone is still 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 a clean line. Don’t touch the bead for 24 hours and don’t run the shower for at least 48 — Selleys Wet Area cures fully in 7 days but is shower-ready at 48 hours. Read your specific tube — different products have different cure times. The first shower will release moisture vapour which actually helps the silicone finish curing.
When to call a tradie
Re-siliconing a clean wet area is a homeowner job. The moment you suspect water has been getting through the bead and reaching the substrate (soft, springy floor near the bath, swelling or staining on the timber frame visible through an inspection access, peeling paint on the ceiling below), the membrane underneath has likely been breached and you’ve crossed into licensed waterproofing territory. AS 3740 says anything that breaks the waterproof membrane on a real bathroom is licensed work, and remediating a failed membrane needs a Class 6 waterproofer with the right paperwork. Don’t try to patch the membrane yourself — it’s a guaranteed leak callback.
Common screw-ups
- Skipping the bleach soak. Mould kill step missed = new bead is dead the day it’s applied. Six months later, black spots, repeat callback.
- Wrong silicone for submerged joints. Selleys 401 in a shower will fail; use Wet Area 3-in-1, Sanisil, or Bostik Wet Area.
- Bead pulls away from one side. Substrate flexes under load — typical with acrylic baths. Add a bath-support strip or build up the bath base, then use a high-modulus sanitary silicone.
- White silicone yellows in 6 months. Either the wrong product (paintable acrylic looks like silicone but yellows fast) or repeated bleach cleaning. Use neutral-cure silicone in spaces cleaned with bleach.
- Joint cracks at the corner. Three-sided joints (bath meets wall meets wall) are stress points; tool the corner extra carefully and don’t skimp on bead depth there.
Cost & time
Selleys Wet Area 3-in-1: $14 a tube; a bathroom takes 1–2 tubes. Caulking gun: $35 for a decent one. Masking tape, metho, bleach: under $15. Total spend $50–$70. Time: 90 minutes of work spread across two days because of the overnight dry between mould kill and re-silicone. A tradie call-out for the same job runs $250–$400 — easy money for the time invested, and you’ll do better than most because you have time to be patient.
The mould kill step is the single thing that seperates 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; do it and the bead lasts five years easy. And use AS 3740-compliant silicone for any submerged joint — it’s the same price as the wrong tube and it’s the legally correct product under the Plumbing Code. Twenty years on the tools and I still see the same shortcuts every week; the houses where the owners did this properly are the ones I never get a callback from. Do it once, do it properly.
