Resealing a bath or shower without the mould coming back

Why the mould keeps coming back

Most callouts we hear about are the same complaint: the silicone around the bath or shower tray was resealed a few months ago and the black mould is already back. Nine times out of ten the problem is not the sealant, it is what happened before it went on. A bead applied over old sealant, over a damp joint, or over surface still carrying soap residue will fail early, and once mould gets a foothold underneath it keeps coming back through a fresh bead no matter how many times you go over it.

Resealing properly means getting the joint back to bare, clean, dry material first. Skipping that step is the single biggest reason a reseal job does not last.

Strip the old sealant right back

Cut along both edges of the old bead with a sharp blade or a silicone removal tool, angled in against the sealant rather than the tile or bath edge, and peel it out in one piece where you can. Any residue left behind, especially in the corner where bath meets tile, gives the new bead something uneven to sit on. Go over the joint with methylated spirits or a proper silicone remover, not just water, since soap film and old sealant residue do not wash off with water alone.

Check the joint is completely dry before you go further. Even a joint that looks dry can be holding moisture behind it, especially in a shower used daily. Give it a few hours with an extractor fan running or a window open, longer if the bathroom does not ventilate well.

Silicone is not grout, and the joint decides which one you need

This trips people up constantly. Tile grout is a rigid, cement-based fill meant for flat runs of tile where nothing moves. The joint where a bath, shower tray or sink meets tile is a movement joint: the bath flexes slightly under weight, the building itself moves a small amount, and a rigid grout in that gap will crack. That crack is exactly where water gets behind the tile and mould starts again, no matter how good the reseal above it looks.

Joint	What it needs	Why
Bath or shower tray edge against tile	Silicone sealant	Flexes with movement, will not crack
Internal corner, tile meeting tile	Silicone sealant	Corners move independently of flat runs
Flat tile-to-tile joints on wall or floor	Tile grout	No movement, needs a rigid fill

Atlas Elastic Sanitary Silicone Sealant is built for the first two rows in that table. It stays flexible for the life of the joint instead of skinning over and cracking the way a cheap general-purpose sealant does after a winter or two. If you are regrouting tile at the same time, keep ATLAS Elastic Grout 5kg for the flat tiled runs and the silicone for every joint where two surfaces meet at an angle or where one of them can move.

Applying the new bead

Mask off both sides of the joint with tape if you want a straight, even line, particularly on a wide gap. Run the bead in one continuous pass rather than stopping and starting, then tool it smooth with a dampened finger or a sealant profiling tool before it starts to skin. Pull the tape away immediately after tooling, while the bead is still wet, or you will drag the edge.

Leave the room ventilated and avoid running the shower or bath until the sealant has properly cured. Cure times vary with the product, the bead thickness and how warm and dry the room is, so check the datasheet on the tube rather than going by how it feels to the touch.

When resealing is not enough

If the substrate underneath the old bead is soft, discoloured, or the tile backing board has clearly been wet for a long time, a fresh bead of silicone on top only masks the problem for a while. That points to a tanking failure behind the tile, not a sealant problem on top of it. Sopro Tanking Kit 5kg covers a standard shower area if you are stripping back to the board and redoing the waterproofing properly, and Waterproof Sealing Tape 1m is what goes into the corners and upstands under that system before tiling goes back on. For a full wet room floor rather than a shower enclosure, Sopro Tanking Kit 20kg covers the larger area. Browse the full range in Waterproofing if you are unsure which kit size the job needs.

Send us the joint lengths and we will sort the list

Measure the total length of bath, shower and corner joints you need to reseal, and tell us if you are also stripping back to tanking or just doing a straightforward reseal. We will put together the right list, sealant, grout, tanking or all three. The full range, along with the rest of the Tools & Accessories you will need, is in stock, and in-stock orders go out within 7 working days. Delivery is free in Ballincollig and Cork City, and collection is by appointment only.

Call 085 868 8459 or email sales@all4building.ie and we will get the list sorted.