the best is an thin coat of epoxy (solvent thinned) as a primer and sealer - providing a stable surface for a few coats of traditional spar varnish. Win-Win the epoxy stabilizes the surface, the varnish provides color gloss and uv protection.
A two part lpu with uv blockers (there are a few of these around) can optionally go over the varnish - protecting the organic varnish
Paul Oman - MS. MBA
A.K.A. “Professor E. Poxy”
Internet Epoxy Confederation (IEC) Homepage
epoxies since 1994
Member: NACE (National Assoc. of Corrosion Engineers) -- SSPC (Soc. of Protective Coatings