your best bet once it starts peeling is to find a shop that will wet sand them and re-coat them before they start to get seriously damaged from UV (plastic gets extremely foggy and turns yellow)
make sure that they are re-coated with something that does have UV protestant in them, otherwise the repair wont last too long.
likewise, you could wet sand them your self and re-coat them, but be warned, its very time consuming and labor intensive.
here are the results from my Lincoln headlights that I redid last summer.
after 400 grit
after 800 grit
after 1200 grit
after coating with a 50/50 mix of spar urethane with UV protectant and mineral sprits
all done with about 5-6 hours of hand sanding per light (I did use a palm sander for the first 400 grit)
also pro tip, don't bother with the 3M headlight repair kit, its kinda expensive for what you get and doesn't come with enough sand paper to really do a good job on both lights.