How to Re-caulk a Bathtub and Shower Without Mould Coming Back
Strip failed caulk, kill the mould behind it, dry the joint properly and lay a clean 100% silicone bead that stays white and watertight for years.
Black caulk around a tub is rarely a cleaning problem; it is a joint that failed and let water behind it. New caulk over old caulk fails the same way within months. The cure is removal down to bare surfaces, a real drying period and a sealant designed for wet areas, applied as a single continuous bead.
Use the right product. Acrylic latex caulk with silicone added is paintable and fine for trim, but it is not for the tub-to-tile joint. That joint needs 100% silicone rated for kitchens and baths, which stays flexible, resists mildew and bonds to glazed surfaces. It cannot be painted, so choose the colour, usually clear or white, to suit the fixtures.
Before sealing, fill the tub. A steel or acrylic tub drops slightly under the weight of water and a bather. Caulking with the tub full stretches the joint open, so the cured bead is compressed, not torn, when someone steps in.
Measure the project
Add the tub-to-wall joints on three sides plus the tub-to-floor joint and any corner joints in the surround. A standard 60 in alcove tub needs about 15 to 18 linear feet.
A 300 ml cartridge lays roughly 25 to 30 linear feet of 1/4 in bead. One cartridge covers one tub with spare for the vanity.
Measure the gap. Up to 1/4 in is a straight bead. Wider gaps need backer rod first so the silicone bonds to two sides, not three.
Plan 24 hours with no shower use after stripping, and another 24 hours after sealing, before the first use.
Shop the material list
Choose products that match the exact room conditions and manufacturer instructions. These catalogue picks give you a practical starting point.
Have these ready
- Utility knife with fresh blades
- Plastic razor scraper
- Caulk removal tool or old chisel
- Isopropyl alcohol and lint-free rags
- Household bleach or a mould cleaner
- Hair dryer or small fan
- Painter’s tape
- Caulking gun
- Spray bottle of water with a drop of dish soap
- Nitrile gloves
- Paper towels
Build it in the right order
Remove every trace of the old caulk
Slice along both sides of the bead with the utility knife, keeping the blade flat against the tub and the tile so neither is scratched. Pull the bead free in long strips, then scrape the residue with a plastic razor. Where grout meets the tub line, saw out any cracked grout. Do not leave a thin film anywhere; silicone will not bond to old silicone.
Kill the mould and clean the joint
Apply a bleach solution or mould cleaner into the open joint and leave it for the time on the label. Rinse, then wipe with isopropyl alcohol to remove soap film and body oils from the surfaces either side of the joint. Alcohol also pulls moisture out of the gap.
Dry the joint completely
Leave the bathroom unused for 24 hours with the fan running, or speed the process with a hair dryer along the joint. Trapped water behind the bead is the usual cause of early failure and the black line that returns months later.
Fill the tub and mask the joint
Fill the tub to the overflow. Run painter’s tape along the tub and the tile, leaving a gap of about 1/4 in centred on the joint. Press the tape edges down hard so silicone cannot creep under. Tape every joint you will seal, including the inside corners.
Lay a continuous bead
Cut the cartridge nozzle at 45 degrees to a hole slightly smaller than the joint. Starting in a corner, push the gun along the joint rather than pulling it, keeping steady pressure so the bead fills the gap and contacts both surfaces. Do not stop mid-wall; release the pressure only at the corners.
Tool and strip within minutes
Mist the bead lightly with soapy water and draw a gloved finger along it in one pass, pressing the silicone into the joint and leaving a slight concave profile. Immediately pull the tape away from the joint at a low angle before the surface skins, usually within 5 to 10 minutes of laying the bead.
Cure before the first shower
Leave the tub full and the room undisturbed for 24 hours, longer in a cold or humid bathroom. Drain the tub only after the sealant has cured. Run the fan during the first showers to help the joint stay dry while the silicone reaches full strength.
Common mistakes
Caulking over old caulk
New silicone does not bond to cured silicone or to soap film. Remove everything, then clean with alcohol.
Using paintable acrylic caulk at the tub
It shrinks, cracks and feeds mould. Wet-area joints need 100% silicone.
Sealing an empty tub
The tub drops under a bather and tears the fresh joint. Fill it first.
Leaving the tape on until the silicone cures
Cured silicone tears along the tape edge. Pull the tape while the bead is still wet.
Tooling with a dry finger
Dry fingers drag silicone out of the joint. Soapy water lets the finger glide and leaves a smooth surface.
Project FAQ
Should I use clear or white silicone?
White suits a white tub and white or light tile. Clear or translucent disappears against coloured tile, glass and chrome, but shows any residue left behind it.
How often should tub caulk be replaced?
A properly installed silicone joint lasts five years or more. Replace it when it pulls away from a surface, turns black inside the bead or lets water through, not just when it looks dull.
Can I shower before the 24 hours is up?
Water on uncured silicone weakens the bond and can leave a permanent cloudy surface. Wait the full cure time on the cartridge.
What about the gap where the tub meets the floor tile?
Treat it the same way: strip, clean, dry and seal with silicone. That joint moves with every step and should never be grouted.