I've seen a lot of posts lately where people are using headlight kits, so I figured I'd share my method.
This spar urethane is as good as it gets without making a large investment into UV cured coatings, you can generally get 2 years out of it. Do at least three coats, any less and the longevity drops off.
I'm very excited about a new product from Optimum Polymer Technologies. They're releasing a new coating for plastic headlight lenses, similar to opticoat but with UV inhibitors and acrylic for added flexibility to account for temperature changes. 3-5 minute cure time, easier application. If it's as good as Opticoat, it will be my new coating for lights.