Now this answer I'm pretty confident in. Varnish or oil, or even oil based stains will prevent the epoxy from bonding with the wood, causing delamination, which is the bane of all composite construcion. And beyond this, I wouldn't think that varnish would have any postive affect to the wood under the epoxy. It really doesn't bring out the grain of wood any better than epoxy does, though it does add that nice yellow tint, but that can be added on top of the epoxy.