I'm doing some repairs to my Catalina 36, which is made from polyester
resin/glass over wood (like most "fibreglass" boats). Should I stick with
poly
resin, or go with epoxy? I know epoxy is "better", but I'm concerned that it
wouldn't stick well to the existing poly, and that the (poly?) gelcoat won't
stick to it.

Suggestions?

Lloyd,

Based on my 30 years (mostly as an amateur repairing my own and friend's boats,
a few summer jobs and one "inbetween serious IT employment" stint at Waterat,
probabably the best small high performance boat builder in the galaxy"), I
would NEVER use polyester resin to do a repair. Repair with epoxy. Sand the
area you are repairing well and the epoxy will stick to it much better than
polyester will.

As for gelcoat (polyester) sticking to epoxy. It works for me. Just build up
the repair with epoxy and whatever glass, Kevlar, carbon fiber, etc.
reinforcements you intend to use, level it with epoxy and filler, sand with 60
or 100 grit and gelcoat it. Hint - you can tint the last epoxy layer with some
of the gelcoat, so the substrate is close to the color of the gelcoat. Don't
use much gelcoat for this, just enough to color the epoxy resin, and don't
bother catalysing it. Always mix fillers, tints, etc. into epoxy AFTER mixing
the resin and hardner.

Alexander "Ali" Meller
505s 7200, 8776, 7080
Albacore 4862
Laser 11166, 173969