Silicone is a consumable and it is supposed to be replaced

Most explanations of sealants stop at the point where it starts to matter. This one carries on.

Why the joint exists at all

Two rigid surfaces that move independently — a bath and a tiled wall, a worktop and a splashback — cannot be joined rigidly without cracking. A flexible sealant bridges the gap and stretches as the two move, which is why it is used instead of grout or filler. That flexibility comes from a material that also holds moisture and biofilm at its surface, which is the trade being made.

Understanding it as a wearing part rather than a finish changes how you feel about redoing it.

What actually makes it fail

A bath filled with water and a person weighs a great deal and drops measurably, so a joint sealed with the bath empty is stretched every time it is used. Filling the bath before sealing, and letting it cure loaded, is the standard trade method and it substantially extends joint life. A joint that is too thin, or applied over an old joint, has little material to absorb movement and tears at the edge.

Live with it a while and the failure appears as a hairline gap at one edge, and that gap then admits water behind the sealant where it cannot dry.

Black growth is moisture, not dirt

The black staining in bathroom sealant is mould growing in the surface of the silicone, feeding on soap residue and body oils in permanently damp conditions. Cleaning removes the surface growth and not the material that has been colonised, which is why it returns within days. Fungicidal additives in sanitary-grade sealants slow this and are gradually depleted, giving a few years rather than permanent protection.

Over years, a room that dries out after use will hold a joint clean for far longer than any product, which makes ventilation the real variable.

Doing it properly is mostly preparation

The old sealant has to come out completely, including the thin skin left in the corner, because new silicone does not bond to old. The surfaces must be dry and free of soap and any silicone residue, and a solvent wipe makes a visible difference to adhesion. A continuous bead applied in one pass, tooled once, gives a better joint than several passes and repeated smoothing.

Curing takes around twenty-four hours before the joint is wetted, and using the shower early is a common reason a new joint fails quickly.

Choosing the right product

Sanitary silicone contains fungicide and is intended for wet areas; general-purpose and frame sealants are not and will grow mould faster. Acetoxy silicones smell of vinegar as they cure and can corrode some metals and mark natural stone, so neutral-cure products are used on marble and mirrors. Acrylic decorators' caulk is paintable and much less flexible, which is why it belongs on skirtings and around door frames rather than around a bath.

Using caulk where silicone is needed, because it can be painted, is a predictable route to a cracked joint.

Taste is not the argument here; how a room performs is.

When it is more than a sealant problem

Water appearing on a ceiling below, soft floor around a bath, or timber that has darkened means water has been getting past for some time. Resealing over that hides the symptom while the structure stays wet, which is how small leaks become expensive. Anything involving suspected water in a floor structure, or near electrical fittings, should be looked at by a qualified tradesperson.

A joint that fails again within months usually means the substrate is moving more than it should, which is a different problem entirely.

The takeaway

Fill the bath before you seal it, ventilate the room, and expect to redo the joint every few years.

The best rooms look like a decision, not a purchase.