I just wanted to point out some details that have not been brought up in this discussion so far. I neither recommend nor will I NOT recommend using varnish to protect plastic. I just want everyone to have *all* the details before they go ahead and use this type of product simply because it's a one way street sort of thing... you can't just wipe it off with some 50/50 IPA and start again once it's cured.
The product we are talking about, "Helmsman Spar Varnish", is a wood finishing product, not a plastic headlight sealer.... that does not mean it can't work in this fashion, just that it was never designed or tested for such use. Who knows... next week we could see that product offered as a sealant for headlights if the Minwax marketing guys find out about this!
As for how long it will last...... I dunno, I've never used it for this sort of thing. I did use it to protect my oak door sill but it faces North, has 3 brushed on coats, each before the previous coat had fully cured so they are chemically linked and after 8 years it's still looking pretty good. Halogen bulbs tend to put out a fair amount of UV, never mind the UV from the sun so it's something each of us who wants to use it will have to be aware of and accept the results we get without saying "If I'd known, I would have never done this". Remember too that the UV component of the Varnish is there to protect the Varnish, not the substrate...... I have no idea how much UV such a varnish would filter out or even if it works the same way as a UV protection coating.
My personal feeling about this sort of thing is that if the headlights were already that bad that they needed wet sanding and polishing, they're already past their best before date and if you can coax them back into service for some more time, it's a bonus no matter how long.
I use and will continue to use regular synthetic sealants or polymer sealants like Mequiars M20, M21 or even a couple of coats of Collinite wax. If they get badly oxidized, cracked or crazed before I sell the car then I'll do a polish job on them and continue with my usual sealants..... I can wax/seal them on a regular basis, I can wipe the product off and replace it whenever, it keeps bugs and whatnot from sticking as long as I do the coating frequently and the lights are so small it's not much of a chore to do every month or so.
Regards
Christian