Plaster of Paris is a compound of gypsum, a sedimentary rock, and water, that when heated can be cast into a strong mold. Because of its fast-drying properties, it is a popular choice for home improvement and craft projects. However, those very same fast-drying properties can be a problem when excess plaster gets attached to other surfaces. Luckily Plaster of Paris is a highly soluble product that, if you follow the correct steps, can be removed with ease.
Scrape off as much excess plaster as possible using a paint chipper or screwdriver. If the dried plaster is collected on the bottom of a plastic bucket, a few sharp taps on the side of the bucket should dislodge most of the plaster.
Wipe away the dislodged plaster with a wet sponge. Since it can cause clogging, be sure not to let any of the plaster go down the sink drain.
Sand the surface with sandpaper to scrape away more bits of the remaining plaster. This step should be avoided if the surface can be damaged easily (e.g., on glass, metal, wood, plastic).
Soak a towel or cloth in an acidic solution and place it firmly over the plaster. Begin with a less corrosive acid such as citric acid or vinegar. Leave the towel on the surface for at least 24 hours.
Remove the towel and wipe away the bits of plaster using a sponge. If there is remaining plaster, try moving up to a more powerful dissolving agent such as muriatic acid (which can be found at home improvement stores, used to clean tile/grout) and follow the instructions for use on the package.