When I hang an old metal clock on the wall, the metal acts almost like
a pencil and leaves gray lines or scuff marks all over the paint. These
lines won't easily clean off, either. What in the world causes this?
Even if I put Scotch tape over the metal that contacts the wall it
still does this. I've noticed that other metals will do this to paint,
like coins. Is the metal shedding a layer onto the paint? Or is the
paint reacting somehow to the metal, which occurs even through a layer
of Scotch tape? I've searched all over the internet but couldn't find a
discussion of this phenomenon.