Use mild household abrasives
Window manufacturers recommend abrasive cleaners for the toughest glass stains. Apply a mild abrasive such as Soft Scrub, Bar Keepers Friend or Bon Ami to a soft rag and scrub. These products usually won’t scratch glass, but start in a small, inconspicuous spot just to make sure. If elbow grease alone won’t do the trick, or if you have large areas to cover, use a drill and small buffing wheel (Photo 1; sold at home centers). You can use a similar method on glass shower doors (Photo 2). An inexpensive electric buffer works fast on the large surface.