PAINTING PLASTER

Sun (Auckland), Volume IV, Issue 992, 7 June 1930, Page 25

PAINTING PLASTER

NECESSARY PRECAUTIONS HINTS FOR THE WORK An article in the “Decorator” gives the following hints on dealing with the plaster "walls of a newly-completed building. “The plaster has not matured and it contains much free alkali that is certain to destroy the liquids with which the paint is mixed and cause the paint to spot badly. The real remedy Is to let the plaster age for several months, but as a rule the owner demands prompt action in the matter of painting, so the painter is obliged to find other means of killing this causticity. About the best known method is to give the walls a wash with a solution of 41b of zinc sulphate crystals in one gallon of water. Apply this with a distemper brush and allow it time to dry. Before priming, brush the surface to remove as much of the white powder as is possible. “If there are any rough places in the plaster, scrape them to a level surface with a steel scraper. It is not advisable to use sandpaper, because such rubbing will leave porous spots, which are sometimes troublesome. “The next precaution is to fill all the open cracks in the plaster. There will probably be many fine hair line cracks, but these will be taken care of later. It is only the open cracks and blisters that need filling. Blisters form in plaster because of small particles of unslaked lime that absorb water and swell, causing small pieces of piaster to become loosened. These and the open cracks must be cut out clear to the lathing with the point of a putty knife. The cracks should be opened fully a quarter of an Inch, and a little wider at the bottom than on the surface.

“Before filling these cracks wet them with cold water to make them less absorbent and carefully fill them with plaster of paris putty. As clear plaster of paris will set too rapidly 10 work conveniently, it is well to mix it in some way which will retard the setting. Mixing the plaster with glue size or a solution of borax will help to keep it workable for a reasonable length of time. Be careful that the edges of the patches are flush with the surface of the wall. As these patches will be more porous than the balance of the wall, it will be necessary to go over them when dry with a thin coat of white shellac. When the shellac is dry it will have a high gloss, which may show through the final coat of paint, so it is always well to give it a thin coat of flat white paint.

“A good formula for the priming coat on plaster walls is as follows:

1001 b white lead. 7 gallons pure boiled linseed oil, 1 gallon pure turpentine. “The formula will make about 11 gallons of thin, penetrating paint that will form a good solid foundation on which to build subsequent coats. Ir, should be applied freely and allowed sufficient time to dry hard. “When dry, if the paint shows a uniform gloss, this indicates that 'it has sealed the surface aud the second coat can be applied safely. Sometimes, however, plaster walls are so porous that the priming coat cannot seal the surface satisfactorily and fiat spots and hair lines will show through the paint. This condition calls for a coat of some kind of size that will form a thin film over the wall that will stop all suction. “The painter has a choice of many specially-prepared wall sizes sold ready for use, most of which work admirably, but an ordinary glue or varnish size will answer just as well, and is more easily procurable. The use of glue size is criticised by some because of the possibility of moisture in the plaster destroying its hold on the surface. This would be true if applied directly to the plaster, but when it lies between the priming and second coat of paint there is little possibility that water will get to it. “A good dependable job of painting on unpainted plaster walls cannot be done with less than three coats of paint.’’

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