Yes you would need to remove the varnish if you really need to do more work. I am not sure of the brand you use, but spirit varnishes are removed with mineral spirits ( Gamsol for Gamblin Gamvar) or turpentine, for example dammar or gloss varnish, depending on the brand of varnish. You can test by putting a small amount of varnish on on old work or surface. Let it dry and try removing it. Wipe on some solvent gentlywith a lint free cloth, wipe off with another clean lint free cloth. I hope this helps.