Just to get it dry the first time
Let your masters sit in the mold overnight, then take them out and dry in the oven. The last one I made, I mixed about 25 percent elmers glue with the water. IIRC Vodkaman posted this tip to keep the finished mold from being chalky. Make sure the water is cold, mix in the glue then mix in the dry plaster until it is about as thick as cake batter.
After taking the masters out I baked the mold for about six hours @ 200 deg. After it cooled I coated it with Devcon thinned with alcohol. It took three coats, letting each coat dry for at least two hours.
Hope this helps. Show us how it turns out.