There's no magic involved in a re-varnish. If you surface is good but just needs a fresh coat, you can use fine 300 or higher on it. I like wet sanding because there's no dust and the paper does it's work forever. I actually wet sand with my Makita pad sander and a hose. (don't try this at home). Or use a sponge.

If you have a large build up of varnish, you could use a ROS and fine paper to level it and take some off and switch to a pad sander for the final super fine sanding. Always be careful on edges because that's where the sander and grind through the surface very quickly and expose glass. I hand sand my edges like cockpit lips etc.

I have used the open coat paper (The grey stuff) but still like wet sanding of varnish best. You could even go over the surface with 500 grit if your last varnish job is good but old.

The toughest part is starting it.