These belt marks are a pain. My rave I didn't clean right after play and the ball sat in my bag for a few days till I got to it. I was in a rush that day of practice - leaving out of town.
Anyway, I had to use my spinner to help remove the belt marks. I used the Remedy Rx then CnD. It was a task. Because of this experience, I make sure to clean as much as I can and remove the belt marks. (happens when I bowl on the low end of the house.) And since my Micro towel is soaked, I place it over the ball just so those belt marks don't harden, like what Milo stated too.
Side notes:
It's a pain to remove these belt marks, but I like this house. I plan on bowling here next season. It's better than that AMF/bowlmorons I bowl at now.... plus at this other house, it's .25 cents a game for practice. Also, if a team is added to a league, I was told the house gives additional prize money of up to $500.