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Sheetrock® Brand UltraLight Gypsum Base Imperial® are the industrys first and lightest 1/2 in. (12.7 mm) rigid gypsum base for veneer plaster systems. These panels feature an innovative, noncombustible gypsum core that is encased in a distinctive, 100% recycled blue face paper that controls water absorption, provides a strong plaster bond and resists plaster slide or slip under the trowel. The panels also provide a rigid base that resists sag. The face paper is folded around the long edges to reinforce and protect the core, and the ends are cut square and even. The long edges of the panels are either tapered or square, allowing joints to be reinforced and concealed with Sheetrock® Brand joint treatment systems.Basecoat.