I'd say your approach sounds good, IF you wet out any exposed plywood with
epoxy before applying the filled stuff. It doesn't have to set up, it just
prevents the next coat from losing epoxy into the grain.

Heating it first so it sucks the epoxy in as it cools is a good approach.
If your wife's hair dryer is unavailable, Black and Decker makes a hot air
gun which sells for a reasonable price.

For "grinding back" I'd suggest a disk sander or one of the $10 (USD)
gadgets from China. No glossy surfaces wanted.

Roger

http://derbyrm.mystarband.net/default.htm
"David Flew" wrote in message
...
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