Rinse water for window washing

Glass Cleaner / Water Stain Remover

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A. A water softener works by substituting a new salt for the old salt. I don't think it would help at all. A dilute solution of isopropyl alcohol would probably work somewhat, as it does on eyeglasses, by leaving less water on the windows to dry and stain them.

A. Window washers use deionized water as a rinse because it 99.9% free of any impurities which cause water spots. It is almost pure H2O. Soft water has salt in it which is an impurity. Soft water would stain your windows with salt and other impurities as it dried on the window. If you do not want the expense of deionized water, but want to rinse without water spots, another possibility is to use Reverse Osmosis water. You can get this from the local water supplier or water store and rinse away your sudsy window with it. RO water is about 94-96% free of impurities. There is a small possibility that you could have a few water droplet stains left behind.