Ceramic Tile Mastic Multi-purpose white, nonstaining high-performance mastic for installing ceramic tile, ceramic mosaic, and quarry tile over gypsum wallboard, exterior grade plywood, concrete, plaster, glass mesh mortar units, and existing ceramic tile. Recommended for use in interior floors and walls and on interior surfaces exposed to prolonged water exposure such as tub enclosures, showers, countertops. Not suitable for use on exterior surfaces. Nonflammable. Coverage varies due to the substrate, type of tile, and size of notched trowel. On average 1 gallon covers 21 sq. ft.