In conjunction with water, a mixture can be used to prepare a sprayable slurry that hydrates to form a machinable plaster composition. The mixture includes from about 80% to about 98% by weight calcium sulfate hemihydrate, from about 1% to about 5% by weight adhesive binder for adhering said plaster mixture to a substrate and from about 0.001% to about 10% of a polysaccharide. The mixture is made into a slurry by the addition of from 10% to about 50 weight water based on the weight of the mixture.