Resealing a bath or shower
Silicone around a bath has a working life of a few years. It goes black at the edges, lifts in one corner, and eventually lets water through into the gap behind. By the time a seal looks bad it has usually been leaking quietly for a while.
Take all the old sealant off
This is the step people rush, and it is the step that decides whether the new seal lasts. Silicone will not bond to old silicone. Any residue left behind becomes a channel for water.
Cut along both edges with a sharp blade and pull the bead away in one piece where you can. A plastic scraper lifts what remains without scratching enamel. Finish with a solvent made for silicone residue, then let the area dry completely. Any moisture trapped under a new bead will go mouldy from underneath, where cleaning cannot reach it.
Fill the bath before you seal
A bath full of water sits lower than an empty one. Seal it empty and the joint stretches every time someone gets in, which tears the bead within months. Fill the bath, seal it, let the silicone cure fully, and only then drain it. The seal sets at its widest point and stays in compression afterwards.
Laying the bead
Cut the nozzle at an angle, with the opening a little narrower than the gap you are filling. Work in one continuous pass rather than short bursts, keeping steady pressure and moving at a constant speed. Stopping and starting leaves lumps that trap water.
Smooth the bead within a couple of minutes, before a skin forms. A wet fingertip works. Anywhere that stays damp wants a bathroom grade sealant sold as mould resistant, which is not the same product as general purpose silicone.
Give it time to cure
Most silicones are touch dry quickly but need a day to cure properly. Keep the bath or shower out of use for the full time stated on the tube. Using it early is the most common reason a careful job fails.
When the problem is not the seal
If the wall behind the bath feels soft, if tiles move under hand pressure, or if a stain keeps returning on the ceiling below, the water has already got past the seal and into the structure. New silicone over that traps the moisture rather than stopping it. At that point the damage needs looking at properly before the surface is made good.