How to tell when silicone sealent has set?

Lafleur_2

I used homebase brand silicone sealant on sunday night between a window sill and a window (both uPVC?). The silicone should have set by now according to the packet, and on first glance its seemed to, but have my doubts as i can dig my nail into the edges of the silicone and can pull tiny bits off it.

Should properly set silicone be close to impossible to "pull away"?

bobthedambuilder

Silicone sealant never really "sets", it always retains some flexibility, which is why it's used as a joint sealer in gaps where there is likely to be some movement. That is probably why you can dig your nail into it.

And, no it's not "impossible" to pull away, especially in places as you describe, where it's very thin. I assume that the surfaces you applied it to were properly cleaned and degreased (e.g. with meths), as any dirt or grease would definitely reduce the adhesion, as would any moisture.

If you applied it on Sunday, it has almost certainly set by now.

Lafleur_2

Ah, forgive me for sounding stupid, i read a breif article that stated good silicone sealant can only be removed with specialized silicone removing chemicals. Mabye i took that too literally

I've tried to make sure that window condensation was kept under control the duration of the setting time by keeping the window open and my heater running on low. I didn't to any cleaning immediatly before sealing, but i did clean the area with borax a few weeks prior to that point

bobthedambuilder

Lafleur wrote: »

i read a breif article that stated good silicone sealant can only be removed with specialized silicone removing chemicals.

Even silicone sealant doesn't last for ever. I have a shower tray susceptible to mould, and I normally replace the sealant around the edge with the tiles about every 2 years to keep it looking reasonable. I've always been able to get most of the sealant out of a joint by digging away at it to get a bit of a grip, then you can often pull it out almost like an elastic band - it's that flexible.

The chemical remover is OK for a final clean-up and getting at bits which would otherwise take an age to scrape off with your fingernails or a soft plastic scraper. You paint it on, leave for 15 minutes or so, then you can wipe it away to get the softened bits off.

As I said, the better the preparation of the joint, the more chance you have. Degreasing with meths gives me a good surface to apply the sealant to, then leave for at least 24 hours to allow the sealant to cure and skin over. Don't make the mistake (as I have!) of applying over old sealant.

Lafleur_2

What are these meths that you're talking about?

stulaunch

You also get what you pay for with sillicone sealant.
I wouldn't recommend using cheap brands, they mould and break seal quite quickly.
I use dow corning when sealing around wet areas. Screwfix sell it.

mr_vampy

"meths" is methylated spirit

bobthedambuilder

mr_vampy wrote: »

"meths" is methylated spirit

That's it. You should be able to pick up a bottle at any DIY store. It's an alcohol-based solvent, usually coloured with a purple stain, which you can use for many cleaning applications (it's very good for getting sticker glue off bottles and glass). It's also used as a fuel in burners and stoves.