There is a great product for this. I've used it to remove varnish on trim in my former 100 y/o house. StarTen. Originally was Dan Swett's Striper That Works.
I had to get this straight from he manufacturer since it was not sold locally.
Comes in liquid and gel. New formula has longer wet time. The gel dried on me so I painted the liquid then wiped it off with a Bounty towel. This does not raise the grain on wood and does not have to be neutralized.
www.starten.com