Spar or yacht varnish: they are what's known as long oil varnishes configured to be flexible and relatively soft to cope with large wood movement. Better brands tend to be the ones stocked by a boat chandler rather than a DIY store, e.g., International and Epiphanes are considered superior brands.
Some people seem to like it, and some don't. I haven't used it myself so I can't comment. It's not varnish which is what you were seeking information on. Slainte.
