Weatherproof sealant

[metasoma]

Hi, I need some sealant that i can apply between a upvc sill and some flashing. In the past ive used both CT1 and Screwfix's No Nonsense Builders Silicone. Yet both seem to weather after a few weeks - they both seems to go like jelly and eventually fall apart.

Anyone recommend a good sealant for this application which can withstand all weathers? In a previous post someone recommended Screwfix's No Nonsense Lead Sheet Sealant yet this cannot be applied to uPVC.

Thanks

 

[aveatry]

Pictures may help but is it a pitched roof ending close to the window with lead flashing occupying the same mortar line as the bottom of the sill.
If so then I would still go with the lead mastic even if it does not stick to the uPVC because you sort of dont need it to really as that section is very protected by the sill you could put a thin line of silicone in first at the back touching the back of the raked out morter and the underside of the sill and then lead mastic over that the mastic wont like sticking to the silicone but it will stick to the remaining brick and lead.
I have used the screwfix one and can recommend it.

 

[metasoma]

Thanks for the response. So the is a bit more to this but i didn't want to get into it - basically the builder took a shortcut and tucked flashing under the window over the top of the sill. When the weather get's better i plan to rectify it but in the meantime i need some sealant that keeps the rain from going under the window.

 

[aveatry]

Sounds very odd but in that case just carry on with the CT1 until you can sort it properly