I have used rattle can rustoleum over old dope just fine. Wash, then sand to scuff, lightly prime with Rustoleum automotie primer and then color. 2 weeks to cure though.
*You cannot use Balsarite, or probably any other strong solvent paint, on this layer cake as it will cause the rustoleum to dope bond to fail.*
I suspect spray minwax varnish will go over the dope just fine. I have used this over other, not fuel resistant, lacquers but not dope... unless that is what my old wood field box was originally finished with. I had no problems with the field box, whatever it was finished with. The field box was washed repeatedly (it was gross with caked dirt and oil), then well sanded prior to spray varnish.
Phil