HOW TO PAINT MASONRY
1. Potential problems with masonry surfaces
Masonry surfaces include bricks, cement plaster, and in some instances gypsum-based plasters or skim coatings are also used. These surfaces provide specific challenges for the painter.
1. Most masonry surfaces are weak, friable, porous and sandy.
2. Cement plasters are seldom aged long enough to allow the alkaline lime content to neutralise with carbon dioxide from the air. As a consequence the
surface is alkaline.
3. Moisture and damp are often present during and after paint application. This can prevent good film formation as well as cause blistering of the dry
paint. It is good practice to locate and remove all sources of water ingress into a structure before commencing painting. A reputable waterproofing
contractor can provide this service. Promac's comprehensive range of waterproofing products can be used.
4. Efflorescence can cause problems on bricks. This happens when water carries soluble salts from within the brick to its surface. The salts are due to
poor or inadequate firing of the bricks resulting in the formation of soluble salts within the brick (usually sulphates from sulphur contamination).
5. Fading and discolouration sometimes occur when painting cement plaster or gypsum plaster. In the case of cement plaster this is normally due to lime
or alkali attack on the finishing coat. The solution is to use Promac Plaster Primer an alkali-resistant plaster primer as a barrier film between the plaster
and the topcoat. On gypsum, a white patch often develops if the plaster is painted with a waterborne product before the gypsum has been adequately
cured. The white effect is caused by soluble sulphates being extracted into the wet paint. This also tends to bleed through subsequent coatings. The
solution includes ensuring the plaster cures overnight prior to painting, and applying a sealer, such as a quality Bonding Liquid. This will prevent
extraction of soluble material into the finishing coating.
2. Standard surface preparation method:
Preparation is the critical part of any painting project: Poor preparation is the principal cause of coating failure. Cement plaster should be strong and hard. If it is not, this indicates a poor sand/cement/water mixture. Flaking, blistering and efflorescence defects are a result of painting over poor plaster and not indicative of the quality of the paint system.
1. Brush off all loosely bound sand particles.
2. Open out any cracks using a sharp knife.
3. Seal all cracks or surface imperfections using a quality Bonding Liquid. Leave to dry for two hours.
4. Fill cracks or surface imperfections using a quality ready-for-use acrylic crack & skim filler paste. Leave to dry for minimum of four hours or as per
manufacturer’s instructions. Sand the surface smooth and wipe down with a slightly damp cloth.
5. If surface is highly porous or friable, apply a quality Bonding Liquid over the entire surface.
IMPORTANT:
1. If the surface of the masonry (especially cement or gypsum plaster) is friable always use a penetrating sealer such as a quality Bonding Liquid. This will
strengthen the surface and reduce the possibility of coating failure.
2. Surfaces comprised of gypsum-based fillers and skim coatings are generally very weak as these materials have inadequate flexibility and wet strength.
Promac recommends the use of a quality, ready- for-use acrylic crack & skim filler paste for this application.
3. For fibre-cement boards, or lower-porosity surfaces, the application of a quality Bonding Liquid is recommended.
3. Primer selection
1. Top coat: Enamel.
2. Primer: Promac Primer Plaster - an alkali-resistant barrier to protect enamel.
3. Undercoat: Promac Universal Undercoat - aids overall coating integrity.
4. Top coat: Acrylic (water-borne).
5. Primer: Promac Plaster Primer - water-resistant barrier to protect acrylic.
4. Finishing coatings
Use the highest quality that you can afford. It is well known that higher quality paints give the best durability resulting in the lowest cost over the long term. Promac has a full range of finishing coats ranging from matt to medium-sheen acrylics as well as enamels.
GUIDE TO PRIMERS AND UNDERCOATS FOR MASONRY
Surface preparation:
Preparation is the critical part of any painting project: Poor preparation is the principal cause of coating failure. Cement plaster should be strong and hard. If it is not, this indicates a poor sand/cement/water mixture. Flaking, blistering and efflorescence defects are a result of painting over poor plaster and not indicative of the quality of the paint system.
1. Brush off all loosely bound sand particles.
2. Open out any cracks using a sharp knife.
3. Seal all cracks or surface imperfections using a quality Bonding Liquid. Leave to dry for two hours.
4. Fill cracks or surface imperfections using a quality acrylic crack & skim filler paste. Leave to dry for four hours. Sand the surface smooth and wipe off
dust with a slightly damp cloth.
If surface is highly porous or friable, apply a quality Bonding Liquid over the entire surface.
IMPORTANT:
1. If the surface of the masonry (especially cement or gypsum plaster) is friable always use a penetrating sealer such as a quality Bonding Liquid. This will
strengthen the surface and reduce the possibility of coating failure.
2. Surfaces comprised of gypsum-based fillers and skim coatings are generally very weak as these materials have inadequate flexibility and wet strength.
Promac recommends the use of acrylic crack & skim filler paste for this application.
3. For fibre-cement boards, or lower porosity surfaces, the application of a quality Bonding Liquid is recommended.
Primer selection
1. Top coat: Enamel.
2. Primer: Promac Primer Plaster - an alkali-resistant barrier
3. Undercoat: Promac Universal Undercoat - aids overall coating integrity.
4. Top coat: Acrylic (water-borne).
5. Primer: Promac Plaster Primer - water-resistant barrier to protect acrylic.
Promac primer plaster typical coverage rates:
• Non-stipple effects: Synthetic / wool roller.
• Floated Plaster: Promac Plaster Primer) 7m² / litre
• Scratch Plaster: Promac Plaster Primer) 4m² / litre
• Spanish Plaster: Promac Plaster Primer) 3m² / litre
Other important information
1. Never apply paint if rain is forecast, or expected, during the day of application.
2. Apply paint after 08h00 (summer) or 10h00 (winter) to ensure that the roof is not too cold or damp.
3. To avoid excessive roof temperatures and to allow the paint to harden sufficiently before any afternoon rain may occur, painting before 12h00 is
recommended.
4. To reduce colour variations always use the same batch number for touch-up work, which may otherwise be visible on the painted roof.