Even tradded, a window that's been rained on, once dry is still clean. Bird mess, spider mess, dust etc doesn't somehow magically end back onto the windows because it's rained.
I think it stems from when we all had coal fires and air pollution was abysmal. I think the rain picked up the soot and other contaminants on the way down, and it was left on the windows. I mean when you go to Bath, you can see buildings that haven't been cleaned and they're completely black from the sooty atmosphere.
And now it's kinda an old wives tale that a minority of folk will use to miss a clean and save a few quid. I really don't want customers like that. I just drop them, always have, and consequently I'm full up with decent customers.
I'm sat right next to a window that's been rained on umpteen times and it's still clean.
I explain to customers - like I did this morning in the humping rain - that rain water is soft water and won't affect the clean.