This 2 inch x 3 1/8 inch stiff sponge/eraser has been around a long time as a tool used in cleaning surface spots on hats.
Easily wipes away dirt, dust and grime. Can be used on all shades of hats.
Like a pencil eraser, one carefully rubs the surface spot until it "balls up" and can be brushed away.
For best results, use a separate sponge for each different color of hat.