Do you fill the bath before sealing?

Movement matters more than the myth

There is no single “always fill the bath” rule. It depends on the material, the support and that bath’s installation notes.

Acrylic, composite and thin-walled steel — especially on adjustable feet — often sag a few millimetres once there is water and a person in it. Seal it empty and the bead is short: when you fill, the joint goes into tension and tears or lets go of a flank.

A heavy cast-iron bath, well set in masonry, typically moves little. Ballast is then less critical. A shove on the rim to check is useful; it does not replace the maker’s sheet.

If the rim already flexes when you stand in it, leaving water in during cure makes sense. If the feet rock or the frame twists, pack the bath first. Sealant is not a prop.

When I ballast — and when not yet

Typical on acrylic or sheet steel on feet: after a dry clean, fill roughly half to three-quarters with lukewarm water and leave it until the joint has cured through (people often talk about a day, depending on depth and product). Many bath makers ask for that. Follow them if they say so; do not invent it if they do not.

Not yet: cutting out the old bead, degreasing and drying. That is done with the bath empty. Fill too early and you trap moisture on the flank; adhesion fails.

Ice-cold water on a hot day: condensation on the acrylic. Boiling water: steam. Lukewarm, and flanks dry to the touch.

Do not pull the plug the moment you have tooled. The silicone is just starting to skin; if the rim lifts then, you lock stress into a mass that has not cured.

Geometry ballast will not fix

A typical sanitary silicone takes movement on the order of 25 % of the joint width, as an ISO class, not as a miracle. A 2 mm gap and 2 mm of sag: the sums do not work, ballast or not. An elastic bath meeting usually wants about 5 mm width at a minimum, PE backer rod if the void is deep, two-sided adhesion.

How I do it

1. Cut out the whole old joint.
2. Degrease rim and tile with the bath empty, then let it dry.
3. Set backer rod where the gap is deep.
4. Primer only if that substrate needs it.
5. Ballast if the bath type and the maker justify it.
6. Sanitary silicone (OTTOSEAL S100 here), tool it, leave the water still while it cures.

Exceptions

Ballast without geometry is not a miracle. Loose support: building work, not paste. Cast iron sitting well: no need to stage a fill “because the internet said so”.