I notice noone has mentioned this possibility, but it's a situation I'm left with after all the storms. In my area, the salted up windows from the rotten weather a couple of weeks ago, have started to bake on. I thought a good bit of rinsing would shift it, but noticed on one of my pubs. Looked like I hadn't touched it, just glad I'm still checking these newbie custies after the bad weather. Fortunately a good scrub shifted the mess, but it took a while. Since then have found a few more, feels like brushing over sandpaper on some, vaseline on others!!
It was always worst where water comes off an overhang nearby, like a low roof or badly fitted guttering.
Did 6 bungalows today, all baked on, dipped my brush in a TFR solution, came straight off and gleamed like new when dry.