Silicone Sealant Fails When You Rush It

What follows is the working version of applying silicone sealant: the decisions in the order you actually meet them, with the reasoning attached.

What the seal is doing

Sealant bridges the gap between two materials that move independently, which is why a rigid filler cracks there and silicone does not. A bath flexes downward when filled and rises when emptied, and the joint has to accommodate that movement repeatedly.

Water getting behind that joint reaches materials that were never designed to be wet, which is how floors and ceilings below get damaged. Mould growing on old sealant is feeding on soap residue and body oils on the surface rather than on the silicone itself. Once mould has penetrated the material, cleaning it removes the surface growth and not the staining underneath.

Removing the old properly

Cut along both edges with a sharp blade and pull the bead away in one piece where you can, working slowly to avoid scratching. Scrape the residue off completely with a plastic scraper, since new silicone bonds to the surface and not to old silicone. Use a silicone remover product on stubborn residue, then clean the area thoroughly to remove any trace of the remover itself.

Clean with an appropriate solvent to remove soap film and oils, which are the main reason a new bead lifts within months. Let everything dry completely, and in a bathroom that means longer than it looks, since gaps hold water invisibly.

Applying a bead that lasts

Cut the nozzle at an angle to a width slightly smaller than the gap, since you can always increase it and cannot reduce it. Fill the bath with water before sealing around it, so the joint is sealed in its loaded position and does not tear when next used.

Move steadily and continuously, keeping the nozzle in contact and pushing rather than dragging, which forces material into the gap. Tool the bead once with a wet finger or a shaping tool, in a single pass, since repeated smoothing drags the surface and spoils it. Masking tape either side gives a crisp edge, and it must be removed immediately while the sealant is still wet.

Choosing the right product

Sanitary grade silicone contains a fungicide and is formulated for wet areas, which ordinary general-purpose sealant is not. Some sealants are not suitable for certain plastics or natural stone, and the tube states what it can be used on.

By the second month, do not paint over silicone, since paint does not adhere to it, and use a paintable sealant where a painted finish is needed. Check the curing time on the tube, which is usually a day or more before the joint should get wet.

Ventilate while working and afterwards, since curing sealant releases vapour and some types produce a strong acidic smell.

Where sealant is the wrong answer

Sealant is not a structural repair and will not hold two things together that need mechanical fixing. A gap wider than the product's stated maximum needs a backing rod behind it rather than a very thick bead.

Sealing over a leak does not stop the leak, it hides it and lets the water accumulate somewhere you cannot see. Repeatedly resealing the same joint that keeps failing suggests movement or water ingress that needs investigating properly. Sealing around a shower tray that is inadequately supported will fail every time, because the underlying movement is too great.

Leave gas, mains electrics and anything behind a wall to someone qualified and insured.

Keeping it clean afterwards

Wipe the joint down after showering, since standing water and soap residue are what mould grows on. Ventilate the bathroom during and after use, because humidity is the other half of the mould equation.

Cooking for one, clean with an appropriate bathroom cleaner, and never mix cleaning products, particularly anything containing bleach with anything acidic. Deal with small mould spots early, as surface growth cleans off while established growth stains the material permanently. A well-applied and well-ventilated bead lasts years, while a rushed one on a damp surface fails within a single winter.

The takeaway

Take all the old out, get it genuinely dry, one pass with the nozzle and one pass to tool it.

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