metal leaves lines on paint

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.
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An exterior wall? likely the cold is kept behind the clock and condenses a bit in winter, it could be mold or a reaction from moisture and metal. Touch an area with bleach to see if it dissapears in a few minutes, if it does dilute a mixture you might be able to kill it without ruining the paint
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glue some little felt pads to the rear of the clock so it cant touch the wall directly and leaves room for air flow.
hardware stores have these in peel and stick
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White paint ? I've only seen this on white the primary white pigment in paints was white lead I can't remember if it was the white lead or one of the replacement pigments that caused this maybe Titanium Dioxide pigment with aluminum oxides
Spud
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Latex paint is easily marked by metals. Oil paint is much more resistant. The more glossier the surface the less metal will mark it, more textured or matte the more marks from metals.

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