I'm embarking on a campaign to keep the water out of my 1950's boat's cabin
structure. Original construction was plain timber framing, marine ply and
polyester/gelcoat over the top. Rainwater has got in around windows, along
the edges of ply, and various fastenings, both original and additions. I
expect it will take me a couple of years to replace some rotten ply, make
new window frames, and cut out various bits of rotten. I've got some merbau
lined up for window and cabin frames, exterior ply with a couple of coats of
epoxy for flat surfaces. So if I can make it stick, it will presumably
outlast me. But I'm new to this - so any comments welcome.

A typical job is eliminating a vent for a long-gone holding tank. Having
removed the air scoop, cut off the block which kept it above the deck, and
cleaned up the resulting mess, I've got an area about 4" x 8" of exposed and
somewhat damaged (delaminated) 1/2" ply, with 2" hole in the middle. ( I'm
old enough to still be comfortable with the old feet and inches that were
used when the boat was built. ) I've removed all the proud fibreglass
around the original mounting block, love the Fein multitool for this sort of
work ... So now I have a tapered edge of the original glass, then gelcoat,
then many paint layers.

My plan is to clean up a small area of the underside of the ply around the
hole, then prop a piece of HD polyethylene sheet underneath the problem
area. Cut a piece of 1/2" ply to be a loose fit in the hole, perhaps shave
a bit of the thickness so it matches everything else. Coat it with epoxy,
paint the edge of the hole with epoxy / fibre mix, drop it in the hole. If
there are several ply's missing, add shaped / chamfered epoxy coated marine
3-ply pieces to add bulk. Then a couple of layers of glass cloth in
increasing sizes, ( original looks like it had 2 layers, I figure three will
be enough for what I'm doing.) Hopefully by now it will be roughly level,
make it so with epoxy / microspheres mix.

Whilst I'd welcome any comments, my area of concern is how far do I go with
grinding back ( removing) gellcoat. This is clearly not going to be a
strength issue, but I want the patch to stick and to be waterproof under the
paint ...

David