Painting Concrete Blocks

PAINTING CONCRETE BLOCKS The increasing use of concrete building blocks is bringing a new difficulty to the painter, because many people insist that their houses shall be painted at once. Concrete surfaces, however, whether they be building blocks or ce mented walls, cannot be successfully painted until the caustic properties of the cement have become neutralized either by time or by the use of a wash which has the effect of changing the chemical nature of the cement. As very few people are willing to wait for a year or two be fore painting a cement surface, the surface may be sponged with a solution of twelve ounces of oil of vitriol (concentrated sulphuric acid) in one gallon of soft water. This will neutralize any caustic lime present in the cemented sur face, and turn it into the inert sulphate of lime (gypsum). It also serves to roughen the sur face sufficiently to give the paint a better hold. The surface should be well washed with clean water before priming. If the cemented surface is a mouth or two old, the dilute acid wash may be dispensed with, and in its place a solution of two ounces of bicarbonate of ammonia to the gal lon of water may be used, in which case the sur face may be painted upon as soon as it is dry, although it is better to wait a short period of time.

After the above treatment has been given, a simple method of excluding moisture is to prime with pure, well-aged raw linseed oil, giv ing ample time to dry. This should be applied

liberally in order to stop suction, and then a coat of flat paint should be given, consisting of the pigments required to obtain the desired color, mixed with linseed oil, turpentine, and japan drier. If this should dry out unevenly, a second coat must be applied. The work may be finished with any good oil paint or a finishing coat of a good waterproof gloss enamel may be given, made of good pigments and a good quality of exterior varnish.

If the blocks have been self-colored, and all that is desired is a wash that will keep out the moisture without materially changing the color, a wash may be applied to the surface, consisting of one part of water glass (concentrated silicate of soda) to three parts of rain water. This will slightly darken the surface, but will give it a hardness and density that resists the weather and tends to keep out moisture.

The thorough waterproofing of concrete is a subject of considerable complexity, specially treated elsewhere. A large number of water proofing materials and processes have been pat ented and placed on the market in recent years.