: Epoxy and varnish are different animals and have different rules. Epoxy will
: bond chemically if reapplied within 72 hours, (i.e. before it cures).
: After that you will need to scratch up the surface so you get a mechanical
: bond. With varnish there is no chemical bonding, the surface needs to be
: lightly sanded between each coat.

In fact, it is important to let one coat of varnish dry completely before applying the next. Epoxy is catalyzed, and will react and cure given sufficient time and temp. But varnish relies upon air for the solvent to evaporate and to create chemical bonds in the film. If you apply a second coat too soon, the first coat will be deprived of its air supply and will never harden. The first coat is slowest, because epoxy can inhibit the varnish hardening. (This is not entirely a theoretical argument--I had to scrape off a gummy mess when I got impatient.)