If you try to remove the black soot from plastic score reels or the painted
side of playfield plastics using an ammonia-based spray cleaner, you will
only bond the soot more tightly to the plastic and will just remove the
paint. Instead, take some gel toothpaste, mix with water to thin it a bit,
and gently work it into the sooty areas with your fingers or a sponge. The
soot will be picked up by the toothpaste. When the paste gets dirty, gently
clean it off with a wet sponge and your plastic part will look like new.
Repeat as necessary to get all the soot off. After all the soot is removed
you can follow up with a gentle cleaning and buffing using a soft cloth
that is slightly damp with spray cleaner.