In case others are randomly mixing: Polyester and Epoxy do not mix well, either. As a teen, I glassed boards for a shaper, and repaired them for extra money. I accidentaly tried to use polyester resin on an epoxy / polysterene board. It melted a hole through the board.
We did thin with acetone, use release films, and use less catalyst (1oz. resin : 5-6 drops catalyst - compared to 10-15 drops for a "hot coat") to acheive a nice gloss, and we felt that a slow cook (full reaction appx. 24 hrs, but we could accelerate by heating) kept the finished product more pliable.
