Using lightweight Hydrocal and Woodland Scenics rock molds, I can't seem to make rocks without them breaking apart when I try to remove them. Rock molds (such as the enbankment mold) are shallow rocks so there isn't a huge amount of plaster being cast. Have tried to follow mixing proportions exactly, and have tried to remove them after 30-min, an hour later or even overnight. Same result----they break apart. Anyone having "good" success with rocks that I might follow?