Why Mould Comes Back After Resealing (And How to Stop It)
If mould returns soon after you reseal a bath, shower, or sink, the cause is usually trapped moisture behind the silicone — not just surface dirt. Understanding how water gets behind a bead of caulk explains why mould keeps coming back and what to do differently.
The moisture pocket problem
Silicone is meant to bridge two surfaces and block water. If the substrate isn’t perfectly clean and dry, or if old caulk remains, tiny voids form behind the new bead. Water seeps in and becomes a moisture pocket: dark, low-airflow, and slow to dry — an ideal environment for mould to grow out of sight. Even if you wipe the surface clean, the hidden moisture pocket will feed new growth.
What mould needs to grow
- Moisture: lingering water behind the bead or within porous grout.
- Food: soap film, body oils, dust, cellulose from drywall paper, or wood fibres.
- Poor airflow: enclosed corners, deep joints, and constantly damp areas.
Why resealing fails (common pitfalls)
- Old silicone not fully removed: New silicone won’t bond to residue, creating channels for water.
- Resealing over damp surfaces: Even small droplets get trapped and stay wet.
- Wrong product: General-purpose caulks lack mould-resistant additives for wet zones.
- Uneven bead: Thick or lumpy application leaves air pockets and weak spots.
How to reseal properly (step-by-step)
- Remove all old silicone with a plastic scraper/caulk remover. Don’t dig into tub or tile.
- Clean the joint (soap scum & residue) and wipe with isopropyl alcohol to reduce spores.
- Dry completely: Use a fan or leave overnight — rushing here is the #1 reason mould returns.
- Mask the joint with painter’s tape for a straight, controlled bead.
- Apply mould-resistant bathroom/kitchen silicone in a continuous pass.
- Tool the bead (caulking tool or finger dipped in lightly soapy water) to remove voids.
- Respect cure time (often 24 hours) before exposing to water.
Prevention after resealing
- Run the bath fan during showers and for at least 15 minutes after.
- Occasionally wipe down wet corners and horizontal ledges.
- Inspect for micro-cracks or lifting edges every few months.
- Fix small leaks immediately (shower door sweeps, dripping valves, grout failures).
Quick diagnostics (is it likely to come back?)
- Bead looks cloudy or patchy: Possible trapped moisture or poor bond.
- Edges lifting within weeks: Old residue or movement at the joint.
- Persistent black line at the same spot: Hidden moisture pocket behind the bead.
Key takeaway
Recurring mould after resealing is rarely a cleaning issue; it’s a sealing and drying process issue. Remove all residue, dry fully, choose the right silicone, and tool a smooth bead to eliminate voids. That breaks the moisture-pocket cycle and keeps the joint clean much longer.
Further reading: see our practical notes on tools and prep in Silicone Sealing Tips. If you have a persistent problem area and want a second set of eyes, feel free to ask a question.