If you don't like harsh chemicals such as MEK which does an excellent job but you need great ventilation, try using SoyGel. Soybean oil based, syrup like, and removes polyurethane, varnish, shellac, etc with aplomb. No noxious odors but it does take some time to work. Takes a bit longer though. You can get it at Rocklers, Brownells, or even online from them directly. It is expensive but goes pretty far. I have used citristrip and the like and soygel has worked better for me.