Asbestos-cement board, applied the same as plywood, is made of Portland cement and asbestos fiber formed under high pressure to produce a hard, durable, noncombustible material.The natural color is stone gray and the exposed side of the panel has a smooth hard finish. Asbestos-cement board is not harmfully affected by water and, while new, does not require special treatment. After a time, especiall
y in some localities, it may become streaked and spotted from soot and dust under the action of wind and rain, and such stains may be difficult to remove.
To brighten, the surface should be thoroughly cleaned with water and a hard bristle brush, rinsed with clean water, and a resin-emulsion or rubber-solution paint applied if desired.