Properly sealing wooden frame into stone sill

22 Oct 2005
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United Kingdom
I've got a leak in a sill in a wooden sash window - that seems to keep coming back. I actually replaced the timber a year ago because it had begun to rot.

The issue seems to be the interface between the wooden sill/jamb and the stone sill (which sits proud of the wood) Due to positioning it gets a decent amount of alternating sun/rain - which tends to wear out paint pretty quickly.

I've temporarily filled some gaps with some cheap ('no nonsense') caulking, but I realise there are far more expensive sealants out there.

1) Should I tear out the caulking and replace it with something specially designed for outer use? Or is silicone sealant just that (and marketing!)

2) What's the better ordering of paint coats? Undercoat wood, then liberally cover the sealant and stone with masonry paint, then liberally cover the wood and sealant again with exterior wood gloss? Or use the masonry paint as undercoat? Or use the wood gloss to cover the sealant only? I guess the masonry paint is acrylic emulsion but the gloss is oil based. (If it makes a difference, the wood sill is dark green which always seems to blister versus the white magnolia masonry paint)

No repair I've ever done to the window seems to last more than 3 years, so I'm beginning to believe the issue is the sealant shrinking over time. I'm hoping though that an oil based paint would somehow protect against shrinkage...?
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Unless you are concerned there could be damage to the building from any ingress of water during this winter I would suggest you hold off doing any remedial work until all the surfaces are warm and dry. You can buy a silicone product that you can apply when wet if you really need to do something now but otherwise I would wait until the spring and do it on a nice warm dry day when you can thoroughly clean all the surfaces. This will also give you time to source the products you need to do a job that will last.

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