Painting on Cement Surfaces.

Manufacturer and builder 8, 1891

The Wiener Bauindustrie-Zeitung gives some useful information about painting on surfaces plastered with cement. As every one knows, oil-paint on a cemented surface is likely to be thrown off by the formation of crystals beneath it; while, if it stays in place, it usually presents a mottled appearance, owing to the unequal absortion of the oil from the paint by the cement. The common way of remedying both these difficulties has been to wash the cement surface with acid before painting it. This dissolces any crystas that may have formed, and acts upon those spots in the cement which have become slightly carbonated by the action of the atmosphere, so as to restore the absorbent quality, which the formation of superficial carbonate tends to diminish. Where acid is used, it should be sulphuric acid, made very dilute. Muriatic or acetic acid, which are often employed, leave the cemented surface impregnated with chloride of calcium, or acetate of lime, both of which are very deliquescent, and, by keeping the surface damp, prevent proper adhesion of the paint.

A better application even than sulphuric acid, is, however, to be found in carbonate of ammonia. The crystals of the ammonia carbonate should be exposed to the air until they efflorensce partially into a white powder, which is a bicarbonate, and are more suitable for the purpose than the original carbonate. One hundred grammes of the ammonia salt - about a quarter of a pound - should be dissolved in nine quarts of cold water, and the cement surface washed with the solution. As soon as it is dry, the paint may be applied, and will adhere well, and resist the action of the atmosphere. The carbonate of ammonia is best applied when the cement surface is about three weeks old. For preparing surfaces, perhaps older than this, or more exposed to the weather, silicate of soda is sometimes useful. This should be prepared by dissolving the syrupy silicate of soda of commerce in four times its bulk of water. Three coats should be applied, and, after the last, the wall must be thoroughly washed, so as to remove every trace of silicate from the surface, or it will effloresce, and throw off the paint.