Hi Grant
From what I have read it seems that the finishing or sizing used for fibreglass cloth is either compatable with polyester/epoxy or epoxy only. So as long as you use epoxy resin you should be OK either way.
This is not true for mats (such as chopped strand mats) as the binder used to hold the strands together might not be dissolved by epoxy resin.
But don't just take my word for it!!.
Jon

: Need an answer to how to tell if fiberglass cloth is finished for epoxy or
: polyester. The place where I get my kayaks clear coated has 2 big rolls
: (about 100 yards each) one of 50" wide 4 oz. and one of 60" wide
: 6 oz. He says they are just in his way and I can have them. (I will pay
: him something or else I would feel guilty forever). Obviously if they are
: finished for polyester and don't work with epoxy, they are of no use to
: me. But, how can one tell?

: I just epoxied a piece of the 4 oz. to a scrap of plywood and it seemed to
: wet out OK. What should I look for? If this stuff is good, I have got a
: really good deal on cloth. Thanks for any help.

: Grant