Try Eucalyptus Oil - it works really well and is volatile enough to evaporate without leaving any residue - although a wash with vinegar and water would ensure the epoxy was clean enough for another coat of epoxy or varnish.
Here in Australia we can get Eucalyptus oil in the supermarket, but in the US you may need to try a health food shop. An alternative is ti-tree oil. Most gift shops here in Oz use either for removing price tag adhesive residues.