I've done many repairs with epoxy and covered them with gelcoat, with no
problems with curing or adhesion. However, I'm having a problem on one
particular boat. I did a structural repair on a kayak with epoxy and
fiberglass cloth (nothing special about it), but for some reason, the
gelcoat didn't want to cure where it was over the epoxy (I'm using
finish gelcoat, so air exposure is not the issue). I removed it and
allowed the epoxy to cure for an extra couple of days under heat. When I
re-applied the gelcoat, ~75% of it cured fine. However, I still have
three small spots that won't cure properly where it's over the epoxy.
Where it's applied over other gelcoat, it cures fine. It's been ~2 weeks
since the initial repair was made, so I'm confident that the epoxy is
well cured. I've tried using denatured alcohol, lacquer thinner, acetone
and MEK to clean the epoxy surface, but the gelcoat still won't cure
properly in these three spots. I'm using the same products that I've
used successfully many times before.

The only clue to the problem is that the uncured gelcoat takes on a
slightly yellow-greenish cast.

Any ideas?