You can buy bulk packs of melamine foam on ebay for pennies a piece. I bought a 100 pack from china for less than 10 bucks including shipping. The shipping actually cost more than the product.
My wife wondered what the big bag of white foam was until I gave her one to try. Now my cheap magic eraser alternatives keep disappearing from my shop. She used one to scrub tar from her floor at school.
As wikipedia says: The open-cell foam is microporous and its polymeric substance is very hard. When used for cleaning it works like extremely fine sandpaper. On a larger scale the material feels soft.
Rubbing with a slightly moistened foam may remove otherwise “uncleanable” external markings from surfaces.
For example, melamine foam can remove crayon, marker pen, and grease from painted walls and wood finishing, plastic-adhering paints from treated wooden tables, and adhesive residue and grime from hubcaps.
If the surface is not hard enough, it may be finely scratched by the melamine material.
The foam wears away, rather like a pencil eraser, leaving behind a slight residue which can be rinsed off.
It works well at removing my son’s artwork from the sheetrock in our home.