In my own experience, spots left after washing your car, be it on the windshield or paint, is due to the residual chemicals in the water. I use a water filter/deionizer when rinsing my car off and have no spotting on the car. To remove those spots on the windshield, you can to try a windshield polish and follow the instructions carefully. I have used Griots Garage Fine Glass polish with great success in the past. Hope this helps you.
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