Varnish of all kinds is degraded by UV light, paint lasts longer because the UV is absorbed by the pigment so the interior of the paint layer is protected for a time but will eventually fail. That is why(exterior) paint is overpainted every few years and varnish (even yacht varnish) varnished over every year. I can't think of anything worse than having to varnish a fence every year. If you miss a year you will probably have to strip it all off and start again(depending on exposure to sunlight). Stick to a clear or coloured preservative.