[Jonny 87]

It comes off in the cold weather...

That's true aswell. Not really sure how buy it does. I think the sun bleaches them off in time.

[Bungle]

Stained sills are a nightmare at the minute 

[slap bash]

I had one of these last year small dark brown/black spots on the frames and glass. Tried every thing. Nothing worked
Later found it was a fella with  sprayer, spraying wooden fences with  coating. I could have sworn it was fry poo.