Cranial Technologies recommends using rubbing alcohol to clean the helmet. However, I believe I will now only be cleaning the inside foam with rubbing alcohol as mod podge is water-soluble and I think the rubbing alcohol will take the sealer off. I will be asking about this at our next appointment.  Note: I didn't realize there were different types of rubbing alcohol, but Cranial Technologies says you need to use this percentage of Isopropyl alcohol.