OK, they call it "varnish" on the violin family, but it's really a shellac-based polish like French polish. While I suspect that Spaw will give you good advice about how to seal the area you want to paint, and follow up with how to get specialist polishes to finish the job, I'd like to add that reading a library book on how to French polish and then practising with a bottle of Button Polish on a scrap piece of wood will help get the best finish on the instrument.