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First of all I don't know what it will do to the finish on YOUR car but it hasn't seemed to have hurt mine*. I used to take off a bunch of black smudges and stuff from my car, you know where this person or that car bumped mine over the last 16 years and it picked up a litle paint or door trim here and there. Just wet the thing down and lightly rub and right away they're gone!
AMAZING... magic even.
* obviously the eraser is abrassive and a lot of use will remove clearcoat and paint

I suspect it's foam impregnated with little glass beads but that's just a guess

 

[GoClick]

I've heard those clay bars work well, never gave it a shot, figured I would some day but I was in Safeway when I passed the Mr. Clean Magic Eraser (Extra Power 2 pack) for just 4.72, I thought to myself, "How could I not?!"
Hehe nice ad eh?

 

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It also worked really good at cleaning up the window trim but that wass messy because black got everywhere on my hands.