The polyester "goes off" pretty quick so when you're building up a mold on a plug, you can just keep on putting layer after layer, no need to stop and wait hours for the resin to "go off".
Be warned. This is a risky process. Piling the layers on as described can lead to a very fast heat build up which can be dangerous if it gets out of control. Curing generates heat, heat speeds up the cure which creates more heat........ It can get away from you. This type of cure can also lead to the mold shrinking or twisting after the plug is removed.
Ed S