You probably want a can of
air duster. It's what I use to keep my minis cleaned off. I also have a big soft makeup brush that works well. As long as the foam isn't sticky on the models, a good brush and spray should handle it. If you have an airbrush, that works well too, for dusting them. If they are sticky, you could try giving them a light washing, since they're varnished, and I assume it's long since cured. I'd hit the trouble spots with a q-tip lightly.