Never use conventional varnish unless its been thoroughly outgassed over a period of six months or
more. The solvent fumes aren't good for the print; and water-based versions might not consitiute a suitable barrier layer. What you want to use it true shellac, the alcohol-soluble kind. It is a unique product which can't be synthetically duplicated. You could also apply foil tape to the rabbet, but
shellac is simpler.