Yeh, ive been seeing these black lines on pvc window sills etc that ive been working on. Like the black marks your shoes used to make back in the day. Anyway, I havent caught myself doing it, and I cant imagine how I have, but im beginning to think I may have done them. I dont get it though, as I dont run the edge of the brush over the sill. mighty strange. Anyone know or had this issue. Surely they were all there already? but i wonder.