As a follow-up, @johnnyb posts inspired me to
try one of the 3M kits on my wife's Mini Cooper 2006 stock halogen headlamps. This was already knowing that I could buy a pair of Hella replacements at ~$350 (rather than MINI-branded at about the same price each). Oxidation on her lenses I would call "moderate", no obvious yellowing, but enough that the output and pattern would likely be significantly impacted, especially on the upper surfaces. Here near San Francisco, we have a relatively mild climate in terms of UV exposure (37° N) and effectively no salt from either the roads or the ocean. No sand/dust issues to speak of.
There are two commonly available kits from 3M. The 39008 was ~$18 through a local PepBoys. It includes a 3" drill-mounted backing pad and multiple 500, 800, 1000 disks, a single 3000 Trizact pad, a "fingered" foam buffing pad, and a small packet of rubbing compound. I should have looked first, as masking tape cost me another $3, not counting the gas and time to get to a local auto-paint supplier.
The 39084 kit appears to include a small roll of tape, as well as a second packet of rubbing compound, but nearly impossible to tell what is in there from the photos.
First I tried some cleaning/solvent approaches:
* Multipurpose countertop cleaner (Mrs. Myers) -- no noticeable impact or damage
* Isopropyl alcohol, 91% -- no noticeable impact or damage
Elsewhere on these forums, CRC Brakleen in the red and white can was mentioned. Knowing that if I had any at all it was probably from the days before VOC limitations, I checked the
MSDS on Brakleen
* 60-70% acetone
* 5-10% carbon dioxide
* 5-10%
hexamethyldisiloxane
* And under 5% each of a few other solvent and things
OK, polishing kit in hand, try acetone. Some some yellow on the rag, good sign. Not much clarity change over most of the lens. Top of the lens (most sun exposure) clouds over immediately. My guess is that the hard-coat was shot up there and it immediately attacked the softer plastic underneath.
As a result,
I recommend against trying Brakleen or acetone, certainly on older MINI/BMW lenses
Edit: Checking polycarbonate and solvents, ammonia should likely be avoided as well (present in some glass cleaners). See, for example, https://www.calpaclab.com/polycarbonate-chemical-compatibility-chart/
The 3M kit worked
reasonably well. I didn't notice it until after I finished that there were some barely visible (unless the sun hits it right) marks on one lens, in one area, likely from the sanding disk or pad not being absolutely centered on the backing pad. Every rotation of the drill, the "wobble", even if only a mm or so, "goes back" over the direction of movement. The 3000 Trizact result looked like a headlight again with the buffing compound getting rid of the remaining haze. The fingered-foam disk worked well (don't wear black pants when working with buffing compound!) End-to-end, including time to let the drill cool, my arm recover, and a drink, about two hours.
Would I do it again? Let's see in a year how well Aerospace 303 (2 oz. bottle for $7) works in keeping them clear. I'm not touching the lenses on my OE xenon headlights (they've also held up significantly better) until I know the long-term outcome (especially at $750/side to replace).
Be sure to use proper PPE when spraying this stuff
