If you add varnish before the solvents have left the previous coat, it can sometimes (depending on the conditions and the varnish) take a really long time for the coating to cure, can be months, though a week or more is more likely. Best to not rush.

Btw, just because epoxy isn't soft doesn't in any way mean it's not still curing. It can easily take a month to completely cure at normal shop temperatures, will take longer with a cooler shop and it can prevent the varnish from curing.

Bill H.