Jon...
Per your comment, here are a few more options for your water spot problem:
1. Retail product in green pump sprayer package. LIME-AWAY - spray, dwell and wash. Keep surrounding surfaces protected with a moist soapy solution.
2. Buffered wheel cleaner (acidic) - carefully applied to sponge or towel onto spots and rinsed with pH-alkaline (8-9) soap afterward.
Start with a dry window-glass. A wet surface will inhibit chemical activity.
The process is simple chemistry that focuses on the pH scale 1-6 acidic, 8-14 alkaline. Using an offsetting solution will neutralize the residue. Works great on lime deposits from covered parking deck fallout from concrete drippings. Severe deposits require several applications of solution. If multiple applications are required, avoid interim water rinsing that dilutes.
Caution: Heat is a catalyst. Start with a cool surface for safest control.
The windshield problem was probably exacerbated by the windshield being hot, accelerating the chemical reaction time. Toasted the glass.
Suggestion: I recommend that all carwash and detail operations have a supply of litmus paper on hand for a quick check of pH strength. Acids and alkalines both cause damage and many operators appreciate a safe tool for quick feedback. That's what litmus paper provides.
