By SA Paint Manufacturing Association
Preparation is the most important element for any successful painting project. This applies to both raw concrete surfaces as well as previously-painted exterior walls or interiors.
SA Paint Manufacturing Association (SAPMA) CEO, Tara Benn, provides some guidelines for tackling exterior painting jobs – and says that from the outset you need to pick the right weather for any exterior painting job: do not even think of painting in rainy weather.
New plastered walls:
The new plaster rendering must be completely dry before any paint application can commence.
The drying time of the plaster can take weeks and will be dependent on factors such as thickness of plaster rendering, and weather conditions during curing and dry-out phase of the plaster. The typical curing time for new plaster is between three to five weeks (can be longer in cold/wet weather). With high substrate moisture, high alkali levels will also be present, so it is therefore even more important to allow adequate curing/drying. Applying a paint system to damp plaster will negatively affect the long-term performance of the paint system and can cause premature failures such as blistering, peeling and alkali burn.
A good way to test the dryness of the plaster is by taping a 30cmx30cm piece of plastic, ensuring the edges are closed, to the rendering and leaving it overnight. When the plastic is removed and no water is visible on the inside (the side which was facing the plaster), your plaster is dry enough to accept paint. If wet, further curing/drying will be needed.
Another method is by using a moisture meter (refer to paint manufacturer for acceptable damp levels).
Once you are sure the surface is dry, it must be thoroughly inspected to ensure that no loose materials are present, as this can influence the adhesion of the paint system. A good brooming of the surface is normally sufficient to brush off any loose particles. If plaster spills and other imperfections are visible, it may be required to sand and scrape this off, before brooming. Fill any imperfections with the appropriate filing compound and allow the compound to dry within the manufacture’s specification.
Once the surface is stable (free of any loose /friable material) apply a good quality plaster primer. The plaster primer will aid with adhesion, uneven porosity (sealing the surface) and produce an alkali resistant barrier for the topcoat.
“From the outset you need to pick the right weather for any exterior painting job: do not even think of painting
in rainy weather.”
Normally the drying time of a traditional (solvent base) plaster primer coat is around 18-24 hours. However, water-based options are available with recoating times of about four hours (again consult with the paint manufacturer). Never allow the primer to stand for long periods uncoated – primers must normally be overcoated within 14 days.
Once your primer is dry, apply a minimum of two coats of the appropriate finishing product to ensure a solid, closed, and uniform application. By following these steps, your paint system will give the best possible protection and long-lasting performance.
Previously painted walls:
A previously-painted surface can either be easy to repaint or might require some extra attention to ensure a long-lasting paint finish.
Not all previously-painted surfaces will pose the same problems. Some surfaces or substrates might have a few cracks, while others might have larger or more serious ones. The same applies to dampness: one structure might have a slight damp problem that can be fixed with a damp-treating paint application, while others might require more professional attention such as by a damp-proofing specialist who will use other construction materials to repair damp issues.
All existing loose and flaking material must be removed by suitable hand preparation (sanding and scraping) or a more effective method such as high-pressure water jet. If any fungus or mould is present treat it with a bleach solution and rinse off. Remember to allow the surface to dry completely again.
Ensure the surface is free of all loose material. Rake open non-structural plaster cracks and clean out the crack with a brush. Fill the crack with an appropriate filling compound. Prime the crack before and after filling.
Structural cracks might need some added attention such as re-enforcement or the creation/insertion of a construction joint.
In some cases, the previous paint might have powdered due to natural weathering and therefore it will be required to apply a binding coat such as a bonding liquid (this is generally applied prior to filling).
Once the surface is adequately prepared, primed (if needed), and stable, finish with a minimum of two coats of the correct topcoat to ensure a solid, closed, uniform and long-lasting finish.
(In the next issue: Selecting the right type of paint for exteriors and interiors.)
For further information, contact the SA Paint Manufacturing Association on telephone 011 615 1195 or visit www.sapma.org.za