Go to Wal-mart, they have multi packs of 220 - 320 - 400 grit 3M Wet/Dry (dark gray color) sandpaper for $2.97.

Start with the 400, if after 20 minutes you don't see a lot of progress, move down.

Once you have all the pitting gone, you'll need to pickup some higher grit sandpaper to get it back to mirror shine..

I suggest 600-1000-1500-2000. you could probably skip one of those if you can't find them.. The only place i have found 2000 grit in town was REALLY expensive so I didn't buy it.. and I've managed to get near mirror finishes without it. It just takes a bit more time polishing.