the product mentioned is just 69% cheap solvent and 31% epoxy (check the MSDS). Your spending big bucks per gallon for just over 1 quart of epoxy. make your own - ideally using a high end cycloaliphatic epoxy and maybe 25% solvent.
Two or three approaches. ... multi coats of solvent thinned spar varnish using less and less varnish (the classic traditional approach)
or spar varnish over epoxy - solvent thinned or straight. Epoxy provides a stable base and the varnish provides UV protection - a win win combination.
or - epoxy under spar varnish under two part lpu clear (only a few contain UV blockers).