I had water spots on my windshield (2000 VW Passat, it was only about 6 months old at the time, I believe), that I could NOT get rid of. I had tried everything we had at the house, and even tried #0000 steel wool. Even that didn't get rid of them...it just left nice little streaks when viewed in direct sunlight. Kewl!
Anyway, someone on the VW Vortex detailing forums recommended a product called A-MAZ. I searched the 'net like mad, and finally found it available at a place called Killian Hardware. I don't have the link anymore, sorry...if you can't find it let me know and I'll try to track it down for ya.
Anyway, it was like $7 or $8...I was so desperate that I ended up paying more for express shipping than the product itself! But when I tried it, it was well worth it. With a little elbow grease, it got rid of ALL of the water spots on my windshield. I've used it a few times since, and it has not let me down. I even used it on my side mirrors (that I ended up selling...kindof an odd story if you don't have a Passat), and it got them looking just like new. The stuff smells bad, but it's awesome! It's this really really thick paste. I haven't tried the Zaino Glass Polish, but I've heard nothing but good stuff about it. It's almost sounding like you could be a mobile detailer and bring nothing with you but a PC7424, some sort of abrasive polish, and then the entire Zaino line. Impressive!
Anyway, if you're desperate, I highly recommend A-MAZ.
Gary