I would recommend a urethane sealant. For details see this thread:

http://www.xoutpost.com/bmw-sav-foru...s-re-do-1.html

For pics see this post:

http://www.xoutpost.com/955168-post42.html

The top picture is what the lens looked like when I bought the car. The second one is what the lens looked like after I completed the polishing. The third is the drivers lens after the sanding / polishing but before the application of the urethane sealant. The fourth is the lens after the application of the urethane sealant. In person it looks brand new. And it continues to look that way today...almost a year later.