I'm refinshing one of my first strippers now. I sanded with 220 on a ROS and It took about 45 minutes to do the hull. Varnish is very soft compaired to epoxy or polyester. It sands off pretty easy. You could even go to a 150 grit for starters. The sanded varnish is much whiter than the epoxy and seem pretty evident to me. A final wet sanding would probably clean up any residual varnish.

I'm also finishing up a S & G hull with one year onld epoxy and glass on it. The hardened epoxy is so hard that it wears out sanding discs in a half hour. It's so noticably harder than varnish that I think you'll know when you're there.

I think if you use chamicals, it's tough to not just contaminate the entire surface.