The answers given have to do with boat building assuming since you're on this site thats what it is.

Epoxy will stick fine to dried sanded varnish.. So if you had a desk with varnish on it and wanted to make a rock hard surface, you could do as you asked and lightly sand the varnish and put a coat of epoxy on it. I've done it and it holds up fine, especially if there's a stain and you don't want to screw that up.

But for boat building, you want the intense bonding power of the epoxy on the bare wood or on hardened epoxy (or polyester) that has been sanded.