you'd be surprised how well that works (sanding with lighter and lighter grades of sandpaper) of course with cement I'm not sure how well it will come off...
I just put a nasty scratch on the coanopy of a build last week and in no time flat it actually came back looking better than the original! just started by getting the gouges out, went down to finer and finer grades, and finished with this 'leather' type nail polishing stick I found at Wal Mart... it still had a slight 'hazy' look to it until I re-dipped it in future.. clear as a bell now!