YouTube "presta headlight restoration"
I am a detailer, you can see some of my threads on here for proof, and will be adding this to my arsenal come springtime.
You sand away then this product adds UV protection so it won't turn yellow.
I know Auto Concierge uses it and he is one of the top people I look up to.
2012 BMW 128i 6MT Deep Sea Blue Metallic
FX-R Bi-Xenon Projectors With 5000k Bulbs & Halo's More Kick Ass Than Any BMW