The details don't matter much. Whatever you use has to have UV filters to protect the epoxy. Beyond that it's your choice.
When viewing what's available you can be sure the stuff that costs the most is, usually, superior.

: I recently got the bug to build a kayak so I jumped in and went for it. No
: plans or kit, used the Hulls program to generate the cad files and cut out
: the S&G parts. I now have it glassed and its ready for a top coat over the
: west systems epoxy. The kayak is actually coming out quite decent but I
: don't want to spend fifty dollars a quart for the "best" marine
: varnish. Any suggestions for a good product perhaps a little more cost
: effective (cheaper). How about polyurethanes versus varnish. Thanks in
: advance.