Should I fill the bath with water before sealing around it?

An acrylic bath with 150 litres of water and a person in it weighs the better part of 250kg. That load moves the rim relative to the wall by a millimetre or two. Silicone tolerates compression far better than repeated stretching, so the sealing position matters.

What you'll need

• Enough hot or cold water to fill the bath
• Sanitary silicone and a gun
• Masking tape

Step by step

1. Remove and clean the old sealant completely first.Nothing about filling the bath compensates for a contaminated surface. Cut the old bead out, use a silicone remover, then wipe with solvent and let it dry.
2. Fill the bath to its normal used level.Not to the overflow — to where the water actually sits when someone is in it. That is the load the joint has to cope with.
3. Get in, or add weight, if the bath is on feet.For a freestanding or leg-supported bath, sitting in it while it is full reproduces the real deflection. For a fully built-in bath on a boarded frame, water alone is enough.
4. Apply and tool the sealant with the bath still full.Work along the joint in one pass, mask if you want a crisp line, and tool it concave.
5. Leave the water in while it cures.This is the part people skip. If you drain it after an hour, the joint closes while the silicone is still soft and the whole point is lost. Leave it at least overnight.
6. Drain slowly the next day.Let the water out and check the bead. It should look very slightly compressed, with no tearing at the edges.
7. Judge whether it applies to your bath.A stone or cast iron bath bedded on mortar barely moves; a thin acrylic bath on plastic legs moves a great deal. The lighter the bath, the more it matters.
8. Deal with real movement separately.If the gap changes by more than a couple of millimetres between empty and full, the bath needs securing to the wall — sealing technique will not carry that alone.

Tips

• Cold water is fine and cheaper than hot. The weight is what matters, not the temperature.
• Do the same for a shower tray by standing in it while sealing, and stay there while tooling.
• If the bath has a supporting batten screwed to the wall, this matters far less — the rim is held.

Common mistakes

• Draining the bath before the silicone has cured — The joint closes on soft sealant, which then cures in the compressed position. Every subsequent bath stretches it, and it parts within months.
• Filling the bath but not cleaning the old surface — Silicone will not bond over a residual silicone film however well positioned it is. Preparation beats technique here every time.

If it doesn't work

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