"I'm priming and painting new mediums, and cleaning up used mediums that are painted white with propolis smudges on them from gloves and whatnot. Mineral spirits doesn't work at all. Acetone, fingernail polish remover, works pretty good. Anyone else have ideas?"
jen, household ammonia. get yourself a small spray bottle and mix a little water with it, spray it on, wipe it off. or just dampen a cloth and wipe it off.
ps. household ammonia also works well to clean off hive tools. i use the yellow stuff.