: I kicked around the idea of building a one-off glass kayak. I was thinking of
: polyester, but for as cheap as I can find epoxy now, I'm not as tempted to
: try the polyester for a whole boat.

The two resins are not totally interchangeable. Polyester still is stiffer (less flexible) than epoxy. If you want a boat that does not have a lot of flex to it, you will probably need a layup that adds more glass or other stiffer fibers (carbon?) when using epoxy than when using polyester. Of course the added glass will require more resin, so your boat weight might increase considerably.

Or, you could take an engineering approach and design reinforcements that would give a stiffer hull in the first place. While those would take up some interior space, the loss would not be very much. If his site is still accessible, take a look at the reinforcement that Ray Jardine did to the deck on the composite kayak he built. (I could not get it when I just tried). As I recall there was a pretty clear picture in his description.

PGJ