Can the epoxy be used directly on plywood?

Yes, but you will have to paint a seal coat first which is the same 50/50 mixture that you use to make the epoxy.  It is painted on with a good brush, then you wait 4-6 hrs and you can pour a flood coat on top of it after that.  Make sure to follow the seal coat instructions and mix up a smaller batch than if you were doing a flood coat.