Yinhe Table Tennis Rubber Cleaning Sponge
These sponges are specifically design for keeping your rubbers clean. Soak them in water until they are soft, then sqeeze them out and keep them in the plastic sleeve. To clean your rubber, make sure the sponge is wet enough (but not soaking wet) so that it's not abrasive to the delicate rubber surface. Wipe the rubber until it's clean. Do not apply any plastic protection sheets until the rubber is dry.