Microcement: which silicone, and where it stains

Why it stains

Microcement (the “beton ciré” look) is a mineral render, a little porous, with a protective coat. Ordinary bathroom acetoxy sanitary silicone leaves dark oil halos beside the joint. Plasticisers migrate by capillary action or attack the sealer. You cannot sand that out.

For movement joints — corners, the meeting with the drain — I want a neutral silicone made for natural stone and mineral substrates, without oils that walk. In the wet zone, closed-cell PE backer rod and, often, primer on the flanks: the sealer closes the pore and, without primer, adhesion lets go when it moves (adhesive failure).

When to seal — and when not

Yes when walls and floor are cured and the sealer has had at least about 72 hours — or whatever the applicator of that microcement asked. Then you seal flexible meetings for water.

No if the microcement or the lacquer still smells fresh, or the substrate is not degreased.

Do not fix with silicone a crack in the render. That is movement in the support behind. Smearing paste on a moving coat looks ugly and leaks again.

If the shower corner is already stained or porous from an earlier leak, the mineral sometimes has to be repaired before you reseal.

How I do it

1. Substrate dry, dust-free. Degrease with a residue-free technical cleaner. No washing-up liquid: acids and degreasers in the soap mark the sealer.
2. Backer rod in the corner: stops three-sided adhesion and sets the section.
3. A thin primer on the flanks; respect the flash-off time.
4. Neutral silicone, no migrating plasticisers. Matt if the floor is matt.
5. Tool with a neutral finisher and a spatula. No washing-up soap.

At a stainless or plastic drain, primer on that flank as well: otherwise it lets go under standing wet.

Common mistakes

Acetoxy “shower” silicone on mineral. Sealing before the 2K or wax has cured. Tooling with floor cleaner. Filling a crack in the coat as if it were a tile joint.

Exceptions

Microcement already darkened beside an old joint does not “whiten” with a new bead. A wax sealer and a polyurethane sealer do not behave the same; always wait for that coat to cure, not for a tiling clock.