On my new Coach as well as the one prior the side windows were badly stained by hard water. On the first coach the staining was so bad it really impaired your view.
I tried everything that I could think of and also that had been recommended including vinegar, CLR, ceran cook top cleaner, buffing compound, chemicals and even scotch bright pads. Nothing seemed to do the job without marking the glass. I knew that acids would remove the spots but was concerned about the surrounding areas. I came across a product by C.R. Laurence called Bio- Clean (WSR1) this product worked great on the heavy stains on my old Coach, however I must warn that it is very aggressive and can leave light scratch marks on the glass. On my new Coach I used C.R. Laurence Sparkle (SP101) it was less aggressive and took more elbow grease to apply but worked well on less stained glass, having said that it only took about an hour a window. I would highly recommend these products for removing hard water stains.