If the steel wool is a few days old, and if it was damp, it will rust. Re-using it can make the surface of your art dirty. Try using a fresh pad of steel wool. You can also use a little bit of water to help clean away the dirt and metal residue after using the steel wool. Make sure the panel isn't too wet or the paint will start to abrade. You can always use a quick dab of dish soap and water to clean off the dirt or grey color with a clean paper towel.

Scotch-brite (brand name) pads also work really well to clean tough stains on the surface. The will also knock back the sheen for you if the steel wool is causing too many problems.