Surface Preparation Guide for a Long Lasting Paint Coats
This guide looks at the most common types of surfaces for a finished construction project. Concrete, cement, plaster, brick and stone each have different characteristics and you will need to reference this in conjunction with the sort of exposure the surface will experience. For example the cement walls of a storage closet will not need as much attention to water proofing as an exterior-facing cement wall.
The hardness of plaster varies according to the lime content in it but in general plaster is highly porous. This porous surface reduces the spreading capacity of the paint, especially with the first coat of paint. The easiest type of plaster surface to paint is Spray Applied Plaster because this has a hard uniform surface with less porosity. Fibrous Plaster requires a primer coat because it is highly porous. A dry plaster surface works well with a water-based, mineral silicate or emulsion paint for most coats. If you are unsure it is best to play it safe and use a primer for the best surface lifespan.
Cement and Concrete.
Provided they are correctly applied over uniform substrates, cement rendered surfaces present no decoration problems. Incorrect mixing and application will however cause variable porosity and unevenness of texture, resulting in an unsatisfactory paint finish. Newly-applied renderings are highly alkaline and initial decoration should consist of a solvent based concrete sealer topped with emulsion or water-based paints.
As with plaster, the more porous the surface the higher the need concrete sealer topped with an emulsion and water paints. Pre-cast concrete for industrial buildings has a very smooth surface, equal to that of good plastering, and because of its dryness and low porosity, can be decorated immediately with most types of paint finishes.
You may consider using a pressure cleaner and soft wire brushes to clean off the surface beforehand. Always ensure that brickwork is completely dry before treating the surface (This could take a few days of continuous sunshine to achieve). 'Underfired' common bricks can contain salts which will damage and discolour paintwork applied before the brickwork is dry. Fill in any missing joints of gap before painting. A waterproofing primer is a good idea as a base for brickwork painting.
Stonework is not usually painted, but when decoration is required, ensure that the surface is sound, clean and dry. You may also consider using a biocidal wash to treat and clean any surfaces showing evidence of algae, lichens, etc., before painting.