varnish removal

on interior window trim in 1930's home. looks they used an old painters technique----the gumwood was painted (use of paint) to simulate 'stained wood'finish. then a varnish placed over that. with time/weathering since then----the varnish/shellac has bubbled and also worn away in spots. what is the best way to remove the old varnish without damaging/removing that old paint (simulated staining) work----so that I can replace all the old varnish/shellac with a new coat. Thank you, Respectfully, Fred.