At the moment I am part way through a repair to the hull of a small bit of rot that has gone all the way through. A previous repair job (not mine) of just scrapping out the rot and filling it with epoxy filler has made the situation worse because unlike the wood it doesn't flex and so cracks and then it just traps water. I can't undo all the previous repair but I am having to do a proper repair using some lovely strips of veneers I was given last year (Thank you Mike)
Next job was to cut away the inner and out layer of veneers around the hole.
I did this by scoring through the layers with a sharp Stanley knife and a sharp chisel.
Then shaped the inner layer (slightly beveled edges to help the boding and strength) and middle layer of replacement veneers.
Hopefully the repair will be sound and strong.
Its a bit worrying to cut holes in your boat but quite satisfying to fix the problem. Well I hope it will be fixed ok. We shall see won't we?