How to Waterproof a Plaster Statue for Outdoor Use

Plaster statues are made from a mix of heat-treated gypsum powder and water that hardens when it dries. Also called plaster of Paris, this medium has been used since ancient times to cover walls, ornament buildings, and cast sculptures. With a mold and plaster mix, anyone can replicate sculptures from antiquity, the Renaissance, or more modern times.

Though a plaster sculpture can feel tough, the material is brittle, porous, and slightly water-soluble, which means it can easily be chipped, broken, or stained. Even if you place a sculpture under cover, high humidity can affect the material. Unless you prefer the look of a slowly degrading sculpture, you'll need to seal your plaster statue for the outdoors to protect it from the elements for years to come.

To seal plaster, you can use many of the same sealers that you would use to waterproof wood for the outdoors, such as varnish, lacquer, or polyurethane. If you are planning to paint your statue, waterproof masonry paint is another option. Polyurethane makes a good sealer on a chalky white plaster statue because it causes little yellowing. All polyurethanes create a water-resistant barrier, but only some are formulated to be waterproof. Consider the amount of moisture your statue will be exposed to as well as how glossy a sheen you want when choosing a polyurethane for the outdoors.

When cleaning unsealed plaster, it's best not to use any water. Fine sandpaper can be used for smoothing out a chipped or pitted surface, but it also has the potential to leave scratch marks and isn't typically necessary for light cleaning. Use a gentle scrubbing pad or a white eraser (or both) to clean the surface without causing damage.