took a bit of searching to discover that POP is probably PoP - plaster of Paris and yes plaster of Paris will slow way down if it is not exposed to air - in fact, if you are building up plaster ("the way you would icing") to do sculpting, which is rather uncommon, you keep the set plaster damp in order to add more unset plaster or you have to wet the area where you will be adding plaster "mud"

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