How long does silicone take to dry?

Skin, touch dry and cured are three different things

Silicone sealant doesn't dry the way paint does. It cures, by taking moisture out of the air. Dow's own patent on it describes a skin forming where the sealant meets the air, then thickening from the outside to the core, at a speed set by how fast moisture can work its way in.

So you get three stages. The skin comes first, and the makers say to smooth it and pull any tape before that forms. Touch dry follows: UniBond, for one, puts that at about 20 minutes. Then there is cured, which means all the way through, and that is the one nobody prints as a time, because it depends on the joint.

If it smells of vinegar while it does this, that's normal. Acetoxy silicones, like Dow's and UniBond's sanitary ones, give off acetic acid as they cure.

Why 24 hours is not the same as cured

Here is the arithmetic the tube leaves out. The data sheets ask for a joint at least 5 mm deep (UniBond) or 6 mm deep for a joint 6 to 12 mm wide (Dow). Dow then publishes its own curing figures: 3 mm after one day, 5 mm after three. The rate slows as the skin thickens, because the moisture has further to travel.

So a 6 mm joint built exactly as Dow tells you is not cured right through after 72 hours, even in their lab at 23°C. At UniBond's 2 mm a day, its own 5 mm minimum depth takes about two and a half days.

None of that means the makers are wrong to say 24 hours. By then the outer 2 to 3 mm has cured. It just isn't finished. Dow's own range for full cure on its construction sealants runs from 7 to 21 days and more, depending on temperature, humidity, thickness and how much of the joint the sealant grips.

When can I use the shower?

For an ordinary sanitary silicone, give it a full day, and longer if the room is cold. UniBond's standard sanitary sealant says touch dry in about 20 minutes and fully dry in about 24 hours, and adds that low temperatures and big joints mean longer.

Is 20 hours enough? It's four hours short of what the makers ask for, and their 24 is measured in a warm room. If the bathroom has been colder than 23°C, wait the full day at least. If it's the bath you've sealed, Bostik's data sheet says fill it half full while you seal it and empty it only once the sealant is fully cured. The reason for that is in the guide to resealing a bath.

Then there are the "shower in 1 hour" products. Read the data sheet rather than the front of the tube. UniBond's says the sealant is "suitable for indirect water contact after 1 hour". Its curing speed is still 3 mm a day. So it skins fast enough to take some splashes, but it is not cured in an hour, and nothing on the sheet says it is.

Why isn't my silicone drying after 24 hours?

Five things slow it down, and the first is the commonest.

It's too deep. At 2 to 3 mm a day, a big, thick bead still has soft silicone in the middle on day two or three, even when the skin feels fine. The way to lay one the right size in the first place is in how to apply silicone sealant.

The room is cold. Dow's rule of thumb is that each 10°C rise "might cut the cure time in half". Run that backwards and a bathroom at 13°C, ten degrees under the test conditions, might double it. The data sheets that give an application temperature all start it at +5°C.

The air is too dry. It needs moisture to cure, and Dow says its sanitary silicone shouldn't be used in a totally confined space for exactly that reason. Dow also allows 48 hours instead of 24 before water testing its construction sealants "in colder or dryer environments". So a dehumidifier running flat out in a shut bathroom is working against the job. Give it a day off.

The cartridge is old. GE Sealants says old sealant can go on normally "but then takes too long to skin over, stays soft longer than expected, or never cures into a reliable seal". Unopened shelf lives on the data sheets run from 18 to 27 months, so check the date stamped on the tube. Keeping a half-used one going is a different problem, covered in how to stop silicone drying in the tube.

It isn't silicone. If the tube says caulk or acrylic, none of these times apply. Caulk vs silicone sets out which is which.

Can you speed up silicone drying?

A little, and warmth is the lever. Dow says cure can be sped up "by adding heat and maintaining a moisture source". In a bathroom that means keeping the room warm and not drying the air out, so put the heating on.

Don't point a heat gun at it. Dow says above 60°C is not recommended, because the volatiles in the sealant can evaporate and leave voids or bubbles. Gentle warmth in the room does the job without that risk.

And put on less. Because it cures from the surface in, a bead no bigger than the joint needs is a bead with less in the middle to wait for. If you need a second go over a bead, Soudal says the first one must be completely hardened before the next goes on.