When we do headlight corrections here, after sanding and buffing we're sealing with a ceramic coating.
I believe Internet consensus sis the best option is to sand and finish with a rough grit sandpaper, like 600 or 800 grit and then spray automotive clearcoat over the headlight. Then you basically have the same type of paint on the headlights as on the car body panels.
Re-spraying automotive clearcoat is likely beyond the capabilities of the masses, thus sand, polish, ceramic coat, repeat as needed.
And anytime you can park your car out of direct sunlight - that's optimum and it has an accumulative effect.
More protection from the Sun's UV rays, less fading over time.
More exposure the the Sun's UV rays, more fading over time.