A mixture consisting of joint compound diluted with water, which is spread on a wall like paint (usually on new construction).  It provides a decorative surface, but it is one of the worst enemies of a good wallpapering job.  To install wallpaper over builder’s flat, first wash it thoroughly with a mixture of ammonia and water, and then seal it with an acrylic primer-sealer or a thinned-down oil-based primer-sealer.