gypsum or plaster of Paris spread on a surface to make it suitable for painting or gilding (or a surface so prepared)
source: WordNet 3.0
Plaster of Paris, or gypsum, esp. as prepared for use in painting, or in making bas-reliefs and the like; by extension, a plasterlike or pasty material spread upon a surface to fit it for painting or gilding, or a surface so prepared.
