Use a porcupine scrubber to scrape sap, dried paint or other tricky substances with the glass. Or You need to use a razor blade. Should you neglect your windows for way too long, you're still left with dust and grime which will avert much-needed daylight from pouring into your house. https://dailyspecter.com/achieve-spotless-surfaces-with-pure-water-cleaning-the-ultimate-solution-for-window-and-building-cleaning/