PoP can be a great molding material, it picks up details very well....but it is very slow to dry. If you have a low humidity enviroment and relatively moderate temperatures (70-90 degrees) this time of year, work outside and let it sit for a week or two...it will dry all the way through. I have a mold that I made several years ago and it didn't turn out quite right (missed an undercut) so I left it outside...just saw it the other day under the deck and it's still in one piece, dry as a bone, and got great details (minus the missing part.)
The more liquid you mix it, the better the details and easier it is to release air bubbles....but it also takes a lot longer to dry thoroughly.