Once the factory UV coating has worn off the plastic lenses of your headlights, they'll start to haze over and yellow. Store bought headlight restoration kits provide means to remove the oxidation, but nothing to protect the lenses from UV after you're done... this is why the results don't last and you have to keep buying more kits. The method outlined in this thread will restore the lenses and then provide long lasting protection from UV, years of clarity as opposed to a few months.
Supplies and tools:
400 through 2000 grit wet/dry sandpaper
Bucket o' water
Meguiars M105 compound (or equivalent)
Meguiars M205 polish (or equivalent)
Foam pads, one light cutting and one polishing
Minwax Helmsman Spar Urethane, clear gloss
Heat gun (optional, faster cure time)
I removed some 5+ year old rockblocker film. It left a thick adhesive residue behind, which required removal with a plastic razor and 3M adhesive remover on a rag. This photo was taken after cleaning it up.
500 grit wetsand
I soaked all my sandpaper in a bucket of soapy water and started wetsanding. Remove any scratches, pitting, and remaining factory UV coating in this step.
800, 1000. 1500, and 2000 grit wetsand
Pretty self explanatory. Work the headlight progressing through finer and finer grits, until you hit a minimum of 1500 grit. M105 can handle removal of 1500 grit scratches, but I prefer to hit it with 2000 grit for a better finish.
M105 and a light cutting pad
In this step, I used a Lake Country light cutting pad with Meguiars M105 compound. This removes the scratches from wetsanding and gives the light clarity again. M105 leaves its own swirls and haze, so follow it up with a less aggressive polish and pad.
M205 and a polishing pad
This step removes the haze left behind by the M105, leaving a glass-like surface. After this step, wipe the headlight with isopropyl alcohol to remove any contaminants or oils left on the surface. The lens is now ready for coating.
Mix Helmsman Spar Urethane and mineral spirits 50/50
You don't need much, I could do two sets of headlights with 30ml.
Urethane mixture applied
Fold a clean dry microfiber OR blue shop towel so you have a 1 inch square. Dip it in the mixture and apply the coating using overlapping vertical strokes, then overlapping horizontal strokes. The coating will self level, but watch for air bubbles, runs, and contaminants. If you mess up, mineral spirits are your friend. You can use a heat gun to speed up cure time, and apply as many coats as you like.
Another before/after example