I've used varnish, and I'm planning to switch to polyurethane.

My wood floors have polyurethane on them and they stand up to the wear and tear of sand, high heels, kids and dogs. Varnish would not last that long, so I'd be redoing the floors more often. similarly, I expect to need to revarnish every year or two, while with polyurethane I might go 3 or 4 years. That is a time savings.

While both may scratch, the softer varnish has wide and deep scratches. I'd prefer to have shallow and narrow ones, if I have any at all.

The varnish takes a long time to dry. Eventually it gets plenty hard -- but it does not become the "armor plate" I want to protect an epoxy which is still getting harder and stronger months after it has been applied.

I don't see what difference it makes on how hard the coating is realtie to sanding it off. The only time I'd want to completely remove the finish (varnish or Polyurethane) would be to patch a hole in the glass, and for that a 60 grit sanding belt will remove varnish, or polyurethane just as fast, and the glass cloth barely slows it down. The sharp grit on sandpaper will cut either material. Sticks on a (soft) muddy shore will make an impression (n a pressure mark) in varnish.

For refinishing you don't need to totally remove existing finishes. the sanding simply scrufs them up to provide some "tooth" so the next coat can lock into it.

I see varnish as a weak coating that simply filters out UV. I see polyurethane as an enhancement that toughens the outer layer of flexible and soft epoxy. In both cases, though, these fluids flow into the "scratches" produced by my sanding of the epoxy and by filling them they replace the matte finish with a glossy one.

If I was working with polyester resin I wouldn't use either. I'd use a gel coat. Some people are using polyester gel coats with UV filtering materials over their epoxy/glass work instead of polyurethane or varnish.

Just some thoughts on this.

PGJ