Quoted from jrpinball:

I think the Novus you used over that black schmutz kind of sealed it into the paint, making it very difficult to remove. Getting black off of yellow on a playfield is very tough to begin with. Bad move using Novus before cleaning first. I'd keep cleaning with 92% alcohol and a somewhat abrasive rag or gauze pads until you start getting a dullness to the paint, then carefully use ME and alcohol. Proceed with caution though.