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Slice a cucumber into thin slices and rub them over the glass. It is not instant, and needs to be rinsed afterward, but it does dissolve the hard water residue. Works fine too on chrome faucets that have a slight leak and grow a stone.
When direct sunlight hits a water on a window, it doesn't just dry it, it bakes the residue into the window, penetrating the glass. But it doesn't end there. Every time the sun hits the window, the residue continues to bond with the glass until it is nearly impossible to remove. During the early 1990s, I tried everything from various grades of bleach to steel wool, toothpaste, kerosine, gasoline, paint thinner, you name it, I tried it. Someone even suggested burning it off and all that did was to highlight the stain. I should also add that when I was able to scrub out the stain, there was a film that formed on the glass that could be seen at a side angle.
Then I tried a solution I bought online, gamma-butyrolactone (GBL) used to remove nail polish. All I did was to wipe it on and wipe it off. The stains were gone! No effort whatsoever! A professional window cleaner had been working next door and when he saw my windows, he really believed I had replaced the glass. He could not believe there was any product that could be so effective until I demonstrated how fast and easy it worked.
Today, GBL is only available by mail order from outside the U.S. It is used to clean chrome wheels and it really does get the toughest stains out of metal surfaces.
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