On Jun 5, 8:50*am, stuart noble wrote:
js.b1 wrote:
Wood hardener stabilises the wood and acts as a fixer/primer for
filler.

As mentioned the problem with fillers is they do not expand/contract
at the same rate as wood.

What I've found with sills is there may be an initial shrinkage in the
wood as it dries out after being filled. It may be a couple of months
before this shows as a hairline crack around the filler but, if that gap
is filled, the problem doesn't re-occur.

That is exactly what I suspect.

Never let relatives "flash the pressure washer" at windows to clear
dirt sprayup, the pressure opens any gap between putty & glass right
to the wood creating a water pocket. Obviously such cracked putty
should be replaced, which is best done as a program of replacement
rather than "patch or fix when you find it" :-)

USA do offer flexible epoxy, www.conservepoxy.com/catalog.htm for
example although 1/2 gallon quantities are more suited to timber
buildings. I have met the old structural epoxy, it makes 50yr old
foundation concrete look like Chamberlain.