Has anyone stuck a washing-up pad to the side of their brush for extra cleaning strength?
Don't do it, it will scratch the window.
Are you working near farms or golf courses? This is where its most evident, otherwise known as shotgun fungus. The flys leave spores from their mouths and after a couple of days & a touch of sun, it seems to dry harder than concrete.
I have tried using lemon and fly spray in the mix - too no avail. My scraper usually gets it off, especially with a new blade.