Mix the powder into a paste on a soaked towel, gently rub away the stains, then rinse and squeegee 2 times to get rid of all residue. Challenging water stains, mineral deposits from masonry runoff, and oxidation from steel window screens are common culprits for stubborn window stains. According to https://dailyspecter.com/achieve-spotless-surfaces-with-pure-water-cleaning-the-ultimate-solution-for-window-and-building-cleaning/