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.

Car body filler is probably flexible enough to cope with normal movement
in wood that is in reasonable condition but, in older houses, the wood
is often so porous that it quickly swells by absorbing water.