For this purpose any spar varnish is ok, but the higher the solids content the better. The very best, IMO, is Epifanes but it is insanely expensive. Truth be told I've been using Minwax brand Spar Urethane of late and am liking it. Did a couple stocks with it and am pleased with the results. One advantage of it is you can find it in about every hardware store or big box store.
Regular polyurethane can be used, but I (and a lot of wood finishers I know) prefer good old fashioned spar varnish.
Last edited by gnoahhh; 02/04/16.
