I think that varnish will be fine on its own, there'd be no point in putting anything else over it. I refinished my first bass with polyurethane varnish when I was your age some 40 years ago, and it still seems OK to me...
I might go down to 600grit after the first coat of varnish has dried, but I don't see any point in going as far as 1500. (assuming your grit grades are the same as the UK).
Reply With Quote