I would strip the remainder of failing varnish. use oxalic acid to lighten up the stains. apply a few layers of old timers formula to the wood, then a few layers of spar varnish. followed up by replacing the clear polycarbonate with a clear silicone 50 year caulk. otherwise you could do white 4000 UV as a glazing however you would have to be very carefull to get a perfect bead since its visible from both sides.
