: Can anybody explain how or why spar/exterior varnish works?

I consider the varnish a sacrificial layer. Varnish doesn't seem to like UV any better than anyother material, but it is relatively cheap and easy to apply. So it works by absorbing the UV energy . The UV physically/chemically changes the varnish instead of your epoxy thereby protecting the epoxy. The varnish degrades until it no longer provides protection and you need to apply more. The varnishes that provide more protection and last longer generally have more pigment or color. These just make it harder for the UV to penetrate deeply into the varnish and turn the UV into heat. A nice gloss help reflect the UV so less needs to be absorbed before it attacks the varnish or epoxy.

303 has the advantage of being cheaper and easier to apply than varnish. You could probably do well applying 303 frequently. However, if I remember correctly 303 is not recommended for painted surfaces which would include varnished boats. I'm don't know what will happen to the varnish after long term application of 303. It could chemically effect the varnish and make it crack or something.