To be clear, magic erasers are DEFINITELY abrasive, and it is removing part of the top coat. A magic eraser is basically blown plastic... They work because there are tiny plastic bubbles that break and form sharp edges that work as an abrasive... there are no chemicals in a magic eraser, it is purely a mechanical cleaner.

 

This pinball geek claims his tests show magic eraser is about the same abrasiveness as 1500 grit. Though I question using alcohol as the lubricant for a piece of plastic foam.

http://arcadearchive.com/pinball/magic/