Which sealant should I use outdoors?

The tube that works beautifully around a bath often fails within a season on a window frame. Outdoor joints move with temperature, sit in ultraviolet light and get soaked, and the differences between sealant types matter far more there than indoors.

Safety

Step by step

1. Work out how much the joint moves.A gap between two materials that expand at different rates — timber and masonry, or a large window frame in a brick wall — moves a surprising amount across a year. Tubes state a movement capability as a percentage; for those joints you want 20% or better.
2. Decide whether it needs painting.This narrows the choice immediately. Silicone cannot be painted — paint beads and peels off it. Hybrid and acrylic sealants can. If the joint sits in a run of paintwork, silicone is the wrong answer regardless of its other properties.
3. Use hybrid (MS polymer) sealant as the default outdoors.These stick to almost anything including damp surfaces, tolerate movement, resist ultraviolet, and take paint. They cost more than silicone and are worth it on anything you would rather not redo.
4. Use silicone where the joint is permanently wet and never painted.Around a shower tray, a sink, or a glazing bead. Use an exterior or neutral-cure grade outdoors rather than a bathroom tube — bathroom silicone contains fungicide for indoor humidity, not ultraviolet stabilisers.
5. Use polyurethane where the joint takes physical wear.Ground-level joints, thresholds, anything walked on or knocked. It cures hard and tough. It is harder to tool neatly and less forgiving of a poor cut, so practise on an offcut.
6. Avoid decorator's caulk outdoors.Acrylic frame sealant is cheap and paints well, and that is where its virtues end. It shrinks, cracks and lets water behind it within a year or two outside. It belongs on interior skirting.
7. Prepare the joint properly.Rake out old sealant completely, brush the joint clean and let it dry. New sealant over old, or over dust, will peel away in one strip. This is the single biggest cause of failed joints.
8. Fill deep gaps with backer rod first.A gap deeper than it is wide wastes sealant and cures badly. Foam backer rod pressed in behind sets the depth so the sealant bonds to the two sides only and can stretch properly.
9. Cut the nozzle small and tool the bead.Cut at 45 degrees, starting smaller than you think. Run a steady bead, then smooth it with a wetted finger or a tool within a couple of minutes. Tooling presses it into contact — an untooled bead often is not actually stuck.

Tips

• Buy one tube more than you need. Running out mid-joint means a visible join and a part-cured start.
• Masking tape either side of the joint gives a crisp line: tool the bead, then pull the tape immediately while the sealant is wet.
• Check the tube for a cure time before rain is forecast. Most need a few hours dry, and some need considerably longer.

Common mistakes

• Using bathroom silicone on exterior woodwork — It has no ultraviolet stabilisers, cannot be painted, and often will not stick to timber that has any moisture in it.
• Sealing over old sealant — The new bead bonds to the old one, not the joint, so the whole lot lifts away as a strip the first time the joint moves.

Questions people ask