UV Sealer-Helmsman Spar Urethane by minwax. I had the same problems with my E90 being pitted, and yellow. I wetsanded, polished and added some sealant + wax. It looked good for a week or two then started yellowing again thanks to the insane sun in Hawaii.
Redid my headlights again and added the Urethane as directed in this link: http://www.autogeekonline.net/forum/...lant-idea.html
. Looks brand new! Its UV protected and has a new coat of clear over the headlights! I highly recommend it, not very difficult to apply if you follow directions very precisely.
If you follow the other thread on the linked site, it shows Urethane VS opticoat and the urethane wins!.