Would a spar varnish yellow out and flake too fast? I’ve used some spar varnish on desks I rebuilt for kids years ago.
First coat thinned out I think 50% and subsequent coats less thinning. Built up a nice hard finish that lasted very long time under kids abuse till we got rid of them.
Yep, marine spar varnish would help, but it is amber. Spar is soft, so it flexes. However, any clear finish can break down because UV goes through to the underlying surface. Paint is opaque and blocks UV.
I once sprayed a sign with some leftover clearcoat auto paint. Eventually the wood cracked, but the clearcoat never wore off.