Last year they clarified that ruling about liquids and balls after they've been checked in.
They said specifically that, once the balls had been checked, nothing can touch the ball besides your hands and a DRY towel. To me that means you cannot use anything to clean out your finegr or thumb holes but that same dry towel. Maybe the referees or judges will be more lenient towards you, but I wouldn't count on it,
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"None are so blind as those who will not see."