I know others have posted similar reviews/tips but figure it's always nice to have multiple viewpoints. That's why we pore over reviews, right? This is the 3M Headlight Restoration Kit, P/N 39084, and was purchased at walmart for about $18. This is the kit with a simple foam backer for hand use, not power. I personally feel you get much better results by hand, and just for reference this is what I typically polish on a day-to-day basis by hand at work...
The kit gives you six 500 grit, four 800 grit, one 1000 grit and one disc that isn't clearly marked or described but I'll guess it's probably 1500 or 1800 grit. These are all 3M velcro-backed 3" discs you can get from your local automotive paint supplier or online, but they're spendy. Doing this by hand gets the most out of each disc, especially if you take the time to clean the dry sanding discs frequently as recommended. I used a clean cotton terry towel to wipe the dust from the headlight and then to "pop" the disc while on the backer to remove dust. Play around with the technique to do this effectively. I would encircle the pad with my thumb and forefinger to keep the disc level with my fingers of my left hand while facing it up and then slapped the disc repeatedly with a bunched point of the terry towel in my right hand, rotating the disc as I did so. With this method I used only four of the six 500 grit discs.
Definitely mask the area around the lenses with painters tape and I would suggest masking over the seal if you still have them, not so much to protect the seal but as to prevent rubber clogging the sanding discs shortening their usefulness. The directions are pretty clear but the horn lights present a challenge that with some patience you'll soon master. I used a variety of horizontal and vertical passes with enough pressure to allow the foam pad to deform to the contour of the horn light. Once you get this down you'll figure out how to do the same thing with a circular motion. Again patience and consistency pays off here.
You'll see immediate results when moving to the 800 grit discs and I almost wish I'd had six instead of the four. By the end of the fist two on my first lens (you split the six and four discs between the two lights) I was seriously concerned about having only one each of the wet discs. For water application a simple spray bottle suffices nicely. If you have chlorinated city water I would recommend using bottled drinking or distilled water. I used a 12 once bottle and it was plenty for the entire process. Mist frequently and don't let the slurry dry, remember to mist the pad as well as the lens. Use good lighting in your work area and move around to get different angles on the lenses to see scratches and dull areas you may miss. Time and patience really pays with this process!
Once you're done with the wet discs you remove the discs and flip the pad over to use as a buff with the included polishing compound for final finishing. You're provided with two packets and I used only one with plenty left over. Use about a quarter-sized amount of compound at a time and work in an overlapping circular motion as best you can. Observe your work and when it looks clear you're pretty much done. I used a 5-gallon bucket turned over for a seat while doing this and it was just the right height for me, but a comfy seat makes the difference. I spent about 5 hours start to finish on both lenses. My only complaint with this kit is two additional 800 grit discs and a packet of the finer grade polish compound. I have some at work and it brought the lenses to crystal clear. The kit did beyond my expectation but just a touch more sealed the deal.
Don't worry about any UV coating, film or protectant as you'll simply be wasting your money. There isn't any such thing as a clear UV protectant, simply not possible. There are some UV inhibitors, but they just don't don't last long. If there were such a thing it would revolutionize fabric aircraft covering! The way we deal with UV protection is a coating of aluminum before paint as it presents an opaque physical barrier to protect the fabric. We could save a lot of weight and expense in covering the aircraft if there were such a thing as a clear UV protectant. I'll be happy to discuss this with anyone reasonable enough to do so. So how do you protect those beautifully polished lenses you spent all afternoon on? Simple. Just look at them closely every 6 months or so and when they don't look quite crystal clear spend 15 to 20 minutes polishing with the medium or fine compound. That's it, you're done! No mucking about with any of the UV stuff, just touch 'em up.
Here are the few pics I took of the finished left lens, unfinished right lens, both together and the kit insert paperwork...