When I had my own detailing business, I used a product called Spray Clay. It is made by the Diamondite company which has a complete glass cleaning line to include the Spray Clay (you apply it to the glass and then rub it in with a rough plastic sponge supplied), a Glass and Surface Cleaner and a Rain-X type product.
Their toll free number is 1-800-426-1380. I had very good results with removing acid rain spots from glass, when normal window cleaner would not do the job. The Spray Clay is a natural Kaolin clay formulation dispensed through an aerosol can. The can says it removes Acid Rain spots/ Hard Water Spots/ Paint Overspray and Industrial Fallout from glass and paint.
