[Kilix]

When water and corn starch mix it becomes very sticky - I have a recipe with them I regularly use. Plus, because of how thick corn starch is naturally, I'd imagine getting it inside such a restricting balloon as a punch-ball balloon could be very difficult. I just wouldn't risk it. Besides, one of the primary functions of the outer balloon is to prevent anything from spilling out should it burst (i.e. an added layer of protection). Putting anything inside of it other than the internal balloon would kind of defeat the point.