sorry bump an older topic but figured i'd post it here since it's relevant.
my headlights have already begun to show signs of wear and the coating has actually completely worn off on the upper-side of the headlight bulge.
it seems this is a very high pressure area where the coating along these angled surfaces has completely stripped to the plastic. not to mention my headlights overall have taken a beating from road debris, tire carcass(es), insects, and high pressure washes at the car wash (yes i'm lazy). it was not due to UV or chemical damage as the rear lights still look brand new.
another angle (before):
another angle (after):
I used Chemical Guys headlight restorer starting with 2500 grit and then to 3500 grit wetsanding.
after that i polished in 2 stages starting with their V34, then onto V38 using a dual action polisher (though a simple rotary would be ideal, i had no trouble)
finished off with Blue Magic headlight sealer (which blocks UV). this is not permanent and has to be applied every now and then (on a clean surface) much like other plastic and rubber restoration products. but it also works to help maintain new headlights/taillights to prevent UV damage, won't prevent scratches though.
right now i just applied the sealer which isn't permanent, as i'm contemplating whether to go with an automotive clearcoat with UV protection and buffing it smooth, or simply sealing it with wax and placing a 3M automotive protective film on it such as InvisibleMask (which i've used previously before).
putting a clear on it would make it easier to maintain and polish to a nice clear finish. on the other hand, a protective film would take more of a beating with just the slightest hint of "orange-peel" up close, but i haven't had much luck getting deep knicks and marks out besides replacing them now and then.
i would say if you're experienced with wet sanding or clay bars, shouldn't take more than 30 min. i took my time and finished in an hour including applying the sealant.