Use jewelers rouge. Easy to use and not very expensive at all. You can find it at The Eastwood Company. Comes in powder form in a small container. Just add water. Can be done by hand or TEC has an attachment you place in a drill. I have used both methods, just depends on the extent of the damage.
I think the site may be called window 101 do a search and see what comes up. They specialize in glass cleaning products for compannie's who clean glass for a living including high rise's.Some of the products they sell are used for etched and stained glass from water spots and acid rain..