How to Get Mould Off Bathroom Caulking
Black mould on bathroom caulking (the silicone or latex bead along the tub-to-wall joint and around the shower) is caused by mould species that colonize the porous surface of old caulk. Once mould has penetrated into the caulk material itself (not just on the surface), no amount of surface cleaning will fully eliminate it — the caulk needs to be replaced. Surface mould that hasn't yet penetrated deeply can often be treated effectively.
Method 1: Bleach Gel (Best for Surface Mould)
- Mix bleach with a small amount of baking soda to form a thick paste (the baking soda thickens the bleach so it clings to vertical surfaces)
- Alternatively, use a commercial mould remover gel (HG Mould Remover, Tilex, or similar) — these are pre-formulated to cling to vertical surfaces
- Apply the gel or paste directly to the mouldy caulk, covering it completely
- Place plastic wrap over the treated area to slow evaporation and extend contact time
- Allow 1–2 hours or overnight
- Remove the wrap, scrub with an old toothbrush, and rinse thoroughly
- Ventilate the bathroom during the process
Method 2: Hydrogen Peroxide (Bleach-Free Alternative)
- Apply 3% hydrogen peroxide directly to the caulk — it bubbles on contact with organic material including mould
- Allow 10 minutes of contact time
- Scrub with a toothbrush and rinse
- Less potent than bleach for black mould but safer for coloured grout and surfaces
When to Replace the Caulk
If mould has deeply penetrated the caulk (black throughout the material, not just on the surface), or if the caulk is cracked, peeling, or no longer sealing, replacement is the right answer. Remove old caulk with a caulk remover tool, clean the surface, dry completely (allow 24–48 hours), and re-caulk with mould-resistant silicone caulk. Mould-resistant formulations contain antifungal additives that resist mould colonization for years.
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