Hi, conventional wisdom has it that one cannot apply polyester resin on top
of (cured) epoxy. The amine in the epoxy does not allow for even a partial
cure of the polyester resin. BUT, I know of one boat yard that has used
polyester over epoxy with no problems. I got a blank stare when I asked
about it. I dunno, sometimes you get lucky.
Epoxy should stick and cure over cured polyester so if you really want to
mix and match the two, you may have to plan your build suchly.

Cheers,
Arnold


"CARSON AXTELL" wrote in message
news:2XEBe.22734$O56.21074@trnddc07...
Hey guys/gals--

I hope I'm not bringing up a past controversy or rehashing old ground, but
I was wondering if anyone has any experience successfully combining the
use of epoxy (read "expensive") and polyester ("inexpensive") resins on
the same project? I have not yet found any literature on the subject
elsewhere.

On my first boat, I thought I'd use epoxy on the stitch'n'glue part of the
project, and then switch to polyester resin for the glass lamination of
the bottom of the hull. Since I didn't know what I was doing anyway, I
bought a 1:1 resin/hardener type of epoxy (resin amber, hardener clear)
that comes in quart bottles in some hardware stores, which worked fine for
gluing and taping the joinery. But then, when I used the polyester resin
to glass the bottom, some kind of reaction occurred with the epoxy --even
after several days, the mating surface between the two resins had the
consistency of a runny gel that wouldn't harden. (To the best of my
recollection, the epoxy had already stood for several days in warm
daytime/cool nighttime No. Calif. tempertures of 80F+ to 50F+ and low
humidity of 40% prior to the application of the polyester resin, so it
should have been well cured, or at least set.)

(The question of whether to use polyester resin at all on a plywood boat
is another matter and has yet to be decided, since I understand that
polyester resin, even when fully cured, is not as impervious to moisture
as epoxy. On the otherhand, it doesn't seem to be as affected by UV rays
as epoxy, either, and since I don't intend to leave the boat in the water,
moisture may not be the bigger problem...)

Any knowledgeable insight or advice on the subject would be appreciated.

Thanks,
Carson

PS: The solution to the gel problem was to strip the hull to bare plywood
where possible & start all over using only epoxy--not an experience worth
repeating... Fortunately, the project was just a D4--another testamonial
to the wisdom of making your first boatbuilding project a small one...;}