To waterproof plaster of Paris for outdoor use, it is important to fill in as many surface pores as possible. When dry, plaster of Paris is porous and, like a sponge, will absorb any water or liquid that touches the surface of the plaster. Over time, some amount of moisture will seep through any type of surface treatment applied to plaster of Paris; however, you should still protect your plaster creation from the elements it will be subjected to outside.
Create your plaster of Paris mould or sculpture and let it dry completely.
Coat the plaster with Waterblok or marine resin in a well-ventilated area. These waterproofing agents will not only protect the plaster on the surface, but will also help seal any open pores in the dry plaster. Let dry completely.
Continue to coat all sides with the resin. Put at least two coats on the bottom and top of the plaster sculpture.
Paint your plaster of Paris design once the last coat of waterproofing agent has dried.
Spray clear shellac over the painted area of the plaster to protect the paint from the outdoor elements. Be sure to let the shellac dry before placing the plaster of Paris outside.