General:
With new cement plaster problems arise mainly from three factors:
- The presence of moisture.
- The alkaline nature of some of the surfaces, and the presence of soluble crystalline salts.
- Water is used in large quantities in new structures and until such time as walls, ceilings, pillars, soffits, etc. have dried out, it is dangerous to apply paints which seal the surface and prevent the natural evaporation of the water still deep in the plaster. An impermeable paint will entrap the moisture which will cause blistering and flaking.
Brickwork, concrete and cement plaster are alkaline from the cement and whilst some of the proprietory skim plasters are not intrinsically alkaline, they can become alkaline if the plasterer adds unslaked lime for easier working. Alkaline salts can leach from brick or cement backings during the drying out process. In the presence of moisture, the alkaline salts will attack the binder in oil paints, causing softening (saponification) of the paint.
The surest way of avoiding problems on these surfaces is to postpone painting until drying of cement plaster is complete, but unfortunately this is rarely possible as plaster can sometimes take many months to dry. When decoration must be carried out at an early stage, the paints used should interfere as little as possible with the drying process and be alkali resistant. In such cases “oil” paints cannot be used as they seal the surface and are attacked by free alkali. Fortunately the modern masonry and acrylic emulsion paints allow free passage of moisture in the wall without affecting the paint film. Of great importance is that this is a one-way passage, ie., water vapour can escape through the acrylic or masonry paint film from the wall to the atmosphere but water cannot penetrate the paint film back into the wall.
Another problem for which there is no cure, not even in the case of 100% acrylic paints, is efflorescence, or the development on the surface of a white crystalline deposit, which is due to the crystallisation of soluble salts formed during drying out, and their growth beneath a paint film will force off the coating. Efflorescence often occurs in the following areas: (i) ground level below dampcourse, (ii) lower portion of walls where there is no dampcourse, (iii) below leaking windowsills, (iv) near ceilings from roof leaks and (v) from leaking pipes in walls.
Preparation:
New plaster is not normally subject to cracking initially. Remove dust, dirt, plaster splashes. Clean down generally.
Previously limewashed or distempered:
Remove to bare plaster by wire brushing, scraping or high-pressure water blast. Cut out cracks and imperfections, fill and sand smooth when dry.
Previously oil, enamel, P.V.A. or acrylic painted:
If the existing paint is in poor condition, remove by scraping or with Dekro Super Paint Stripper. Wash down with a suitable concentrated detergent, rinse and allow to dry. Cut out cracks and imperfections, fill with a recommended crack filler and sand smooth when dry.
If the existing paint is in good condition, wash with a sugar soap solution and rinse clean, or with a suitable solvent to remove dirt, grease, wax polish or other contaminants. Sand glossy surfaces with a suitable abrasive. Rinse clean and allow to dry.
Painting:
N.B.: Allow minimum of overnight drying time for an oil and alkyd-based paint to allow for seasonal low temperatures.
