i built a baltic birch 1/2 " ply board in the shape of a spleen door a few years back but never seal it. I want to kite it this summer,what should i seal it with? i don't want it to weigh a ton so varnish or epoxy? and how many coats?

If you don't have anything at home yet and you have to buy one or the other, I would suggest a boat varnish. Usually it is Polyacrillic with additives to withstand all weather conditions and with UV blockers to last longer in the sun. I use yacht varnishyou can see on top of this page.
It is very hard to sand, but that's the whole idea, if you really want to protect it.