glass will be much harder, you will still need to sand down then finish up with a Cerium Oxide glass polishing kit.
basiclly work your way down in Sandpaper to very fine, then polish with the Cerium,
it won't be quick or easy, but it is doable.
best write up I could find:
http://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/143993-diy-fog-light-sanding-refinishing-pics.html