I bought a used 2013 Duramax a few months back. At some point prior to my purchase someone washed the vehicle with hard water and didn't wipe it dry. There were hard water spots all over the paint and glass. The paint wasn't a problem with a good cleaner wax and polish, but the glass was a different story until I found this product:
http://www.driventoperfection.com/products/automotive/driven-extreme-duty-glass-cleaner
I highly recommend this product if you are having trouble with hard water spots on your glass.