I think I read somewhere like in the owners manual not to use cleaners like that on the rear window. But it's up to you. But from experience I once had the hard water spots on my 87 Z and went around looking for a product and tried several. I even had a few detail shops try their methods on my car but no one had success. I ended up finding this product called Diamondite on the internet and theirs worked well and was made specifically for glass. But then later because I ran out of it and it was expensive I ended up just using a turtle wax white rubbing compound and that worked just as well. It must have taken me about 5 hours total to do all my windows because the previous original owner seemed to have never washed the car and those spots had accumulated from acid rain over the last 17 years. Good luck. I hope this helps.
