: Consider fiberglassing the deck with epoxy resin and 1 layer of
: thin cloth to stop the wood from checking. Varnish alone will
: not keep the thin outer layer from developing small splits all
: over it from repeated wetting and drying, but the
: epoxy/fiberglass is stronger than the wood's tendency to split.
: Then finish it with quality varnish with UV shield to protect
: the epoxy from sunlight.

I agree with Dave. I would always put a layer of fiberglass cloth down on plywood if I had the opportunity. It is not "necessary" but will add a lot of strength, stop surface checking, assure a minimum thickness of epoxy over the whole surface and provide an excellent substrate for subsequent coats of varnish.

A layer of 2 ounce/sq yd fiberglass would weight about half a pound, 4 ounce cloth would be about 1 pound, but you would not need as much varnish the so the additional weight would be less.