I had some extreemlly resiliant spots that seeming nothing would get rid of. They laughed at stoners, claybar, Mr. Clean's lime/rust remover, magic erasers w/ vinegar, steel wool, etc.
What finally got them was KAIO via PC using a white polishing pad.
The 1Z does a great job of removing water spots...and also the Duragloss NG (Glass Water Spot Remover) The DG polish is a bit more agressive for really tuff spots....the 1Z and DG are the better glass polishes I have used so far....