@GregK thanks for all the great info! -- I had question about older vehicles with the "dingy" headlights- what is your recommendation? I have tried some kits or had the dealership do it but it doesn't last very long. I do have access to an orbital polisher if that is helpful.... thx
Cloudy headlights are the result of the protective coating becoming oxidized. To fix this issue you have to use abrasive polishes/compounds to remove this oxidation or in more extreme cases, use sandpaper(much higher grit than typical wood sandpaper). Then you have to go through steps of finer sandpaper and then through steps of finer compounds to finishing polishes to properly get the gloss back. Then you have to protect the headlight again with a wax or coating as you have removed the factory protection.
Like scratches on the painted body of the car, you try the “least aggressive product that gets the job done” on headlights. Also like the painted part of the car, you can’t just use an aggressive compound and be done. You have to follow with finer polishing steps.
A decent headlight kit with have various grits of sandpaper(usually wet) either on a hand or round pad attached to a cordless drill, some compound polish and finally a finishing polish/wax with that’s again used on a rotational pad with a drill. Any “just liquid” kit will not get the job done and only be temporary.
Most auto body or car parts shops sell various grits of wet sandpaper on their own as well. 3000 grit(very fine finishing sandpaper) and 1500 or 2000 grit(for heavier haze) is all you should need in most cases.
Open the car hood and tape off the edges of the body surrounding the headlights so your buffer or sandpaper doesn’t hit them. Then start trying less aggressive methods like a compound/polish/scratch remover with your polisher on the headlights. If you’re happy with the results, move to a finishing polish and add some protection and you’re done. Use that polish and then sealant once a year at least to maintain.
If it still looks hazing after the compound on the buffer, then try the wet 3000 grit sandpaper(spray both the headlight area itself and the small square of sandpaper cut from the sheet) and go over the entire headlight by hand with light pressure. You will easily see the white oxidation being removed. If it looks pretty even now(but still dull), you now follow with the compound and then polish on your buffer. The first compound/polish stage on the buffer should transform the clarity of the headlight.
If it’s still not even with the 3000 grit, try the 1500 grit till it’s looking like the oxidation is gone. Then clean and go over the wet headlight again lightly with wet 3000 grit. Then compound then lighter polish then protect.
Severe headlights might require even more aggressive sandpaper which I’d leave to a professional. Sometimes have to do 4 steps of sandpaper on a high speed rotary polisher and then 2 or 3 steps of compounds/polishes on another polisher. More risk that’s not worth it for a DIY.
Honestly rare that a hazed headlight needs more than 1500 grit, 3000 grit, compound and finishing polish. Many times, 3000 grit is more than enough to move to remove the oxidation.