Conservation of render and plaster

Dealing with organic paints

Organic paints are easy to use and abundant. Organic paints give a vast variation in colours and are a fast, cheap and easy way of renovating a façeade. But their chemistry and properties are often not tailored to the mineralic substrate or history of the building.

A non-suitable raincoat

Peeling paint is a common sight on masonry buildings. Old masonry built with lime-based mortars, is dependent on a surface treatment having dampdiffusion open (low Sd value) properties. Lime in the masonry is dependent on CO² to obtain strength. Organic paints may damage the masonry in two ways: The lack of CO² will yield porous renders and sometimes a total collapse of the render. Accumulation of moisture will also result in peeling paints, damages to renders due to dissolution of the lime, and often frost damages. An organic paint may be damp diffusion open, but with a high water repellent effect to act as a raincoat. 

In our experience, frequent painting of such paints gives thick, tight layers that prevent the flow of CO² and cause the accumulation of moisture that penetrates through small fissures. Moisture will get trapped and dissolve the substrate. Most organic paints also cracks with time and opens up for new accumulation. The lime-based substrate is searching for constant chemical equilibrium reacting with both water and CO². 

Remove and replace

Organic paint

A thorough repaint job is achieved by removing all organic paints. Paints can be removed mechanically (sand-blasting, limestone blasting, etc.) or chemically by dissolving the organic paint. Another method is by grinding the surface to remove paint and the uppermost surface of the render. These methods can be used if the renders have a good enough adhesion to the substrate. The choice of method depends on the substrate, amounts of decorations and mouldings, and of course, cost and time. The removal process should cause as little damages to the substrate as possible. Residue will reduce the adhesion of the new paint applied. If the organic paint has damaged the substrate, a full removal of the render might be an option. A normal thumb of rule is the 25% rule. If more than 25% of a flat surface needs to be repaired, complete removal of the render should be considered the more economic option, when compared to a partial repair of the surface. In either situation organic paints should be removed completely. Choice of new paint should be done based on technical and aesthetical needs.