Mabel how big is the ball?
Were you planning to cut it in half later?
Is there any reason why you want to use POP?
Is this like the method doctors use for casts?
I have my own methods of making ball molds so I'm not up to speed on how others do it here.

In the thread about sphere making a week or so ago, I mentioned making a mold with guaze and water putty and said that I would try it. I went round to Wallyworld to get a large ball and found one that only measured 8 inches or so BUT I did find one of those blow-up type beach balls that the label said is 24" in diameter. It was on sale for $3 and came complete with a fabric slip cover that is supposed to make it look like a baseball. Well, when I inflated the thing and realized how huge it is, I went back out to the nearby dollar store and got a smaller blow-up that is maybe 13" or so in diameter and set out to cover it with fabric strips and water putty. I just laid the strips of torn fabric over the ball and brushed on the putty. It took forever to dry and I have now taken it off the ball. The blow-up feature made it easy enough to remove the ball. When I cut around the "equator" I found that the water putty crumbles and the cut edge was not nearly as tidy as I would have liked. I suspect you will find something very similar with the POP. I am going to try sealing the edges with either latex or silicone caulk and will let you know how that went.
One suggestion on separating the two halves--be sure to make a jog of some sort so that the halves are easy to match back together later. It would be virtually impossible to make a perfect sphere with these methods so you really need to be able to put it back together the same way that it came apart.

Good luck....I'll be back
spiderwoman