Water is used in large quantities in new structures. Until such time as walls, ceilings, pillars and soffits have dried out, it is dangerous to apply impermeable paints. These seal the surface and prevent the natural evaporation of the water still deep in the plaster. If the moisture is trapped in this way, blistering and flaking will occur.

The reason for this is quite simple. Brickwork, concrete and cement plaster are alkaline from the cement. While some of the proprietary skim plasters are not intrinsically alkaline, they can become alkaline if the plasterer adds unslaked lime for easier working.

As a result, alkaline salts can leak 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.

Therefore, the surest way of avoiding trouble on these surfaces is to postpone painting until drying out is complete.

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 100% 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 - water vapour can escape through the paint film from the wall to the atmosphere but water cannot penetrate the paint film back into the wall.

Preparation of Cement Plaster Surfaces

New plaster

• New plaster is not normally subject to cracking initially.
  
• Remove dust, dirt, plaster splashes.
  
• Clean down generally.

Previously lime washed or distempered plaster

• 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 or enamel painted plaster
If the existing paint is in poor condition, remove it:

• wash down with a suitable concentrated detergent
  
• rinse
  
• allow to dry
  
• cut out cracks and imperfections
  
• fill with a recommended Crack Filler
  
• sand smooth when dry.

If the existing paint is in good condition:

• wash with sugar soap solution and rinse clean, or suitable solvent to remove dirt, grease, wax polish or other contaminants.
  
• sand glossy surfaces with suitable abrasive.
  
• rinse clean and allow to dry.
  

Painting
Allow minimum of overnight drying time for an oil and alkyd based paint to allow for seasonal low temperatures.

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