I have 3 windowsills that are rotten. One of them has a hole, about the size of 2 silver dollars, that goes through to the stud cavity. The rest of this windowsill, along with the 2 others, is soft enough to sink in a putty knife about one inch.

As far as I can tell, the rot was due to letting the paint flake and chip (rather than a water leak). Once I have the repair done, they'll be properly primed and painted.

Would those wood epoxy hardener/filler kits work in a situation like this, or would I be better off replacing the entire sills? Do the kits strengthen soft wood, or do I have to dig out to solid wood (which in this case, would be digging out most of the sill)? I'm not sure if trying to use an epoxy would just be slapping on a patch and covering up the problem, without actually fixing it.

Thanks for any feedback