Linseed oil paint on exterior plaster/stucco
Linseed oil paint on plaster has a long tradition and was often used on facades in urban environments in the 18th and 19th centuries. Linseed oil paint was considered to last longer than lime paints and also had higher status because it was more expensive. There is little difference between painting on plaster and wood and the working method is basically the same. One important difference to keep in mind is that wood always has a neutral pH of 7, while a freshly plastered surface is alkaline and has a high pH.
Plaster mixtures can contain varying proportions of lime and cement, but generally speaking, the higher the cement content, the faster the curing time. When plaster hardens (through carbonation), the alkaline pH value gradually decreases; after about three months, the plaster surface has a neutral pH of 7.
Linseed oil and linseed oil paint must always be used on a pH-neutral substrate to avoid paint decomposition. Therefore, you should always wait at least three months before painting on a freshly plastered surface. It is difficult to measure the pH of a plaster surface because pH values must be measured in a liquid solution. Painting on old plaster is no problem.
Three-coat painting on new plaster
The plaster must have a neutral pH value (pH 7) to be painted with linseed oil paint. New plaster must cure three months before being painted.
1. Primer/1st coat application
Mix your own primer as follows:
40% volume linseed oil paint, 30% volume raw linseed oil, 30% volume gum turpentine (for example, one quart of base paint is mixed as follows: 12 oz Paint / 10 oz Raw linseed oil / 10 oz gum turpentine)
Apply this primer to the surface evenly and well. Drying time varies depending on the weather. Calculate 3-4 days in hot, dry weather. The surface should feel dry and a little rough when you run your hand over it.
Gum turpentine consists of distilled resin from various conifers and decomposes in nature. It contains terpenes, which are the major constituent of natural essential oils. Gum turpentine should not be confused with white spirits, which is a petroleum product. (For more information on risks, see our safety data sheet).
Alternative Primer, without gum turpentine:
Mix your own primer as follows:
50% volume linseed oil paint, 50% volume raw linseed oil; let dry for at least 4-5 days
2. Intermediate application
Paint with undiluted paint. If the paint seems difficult to spread thinly on a rough surface, you can thin the paint with 10% volume gum turpentine. Let the surface dry for at least 2-3 days.
3. Final application
Finish with undiluted paint.
Painting with Ottosson White Plaster linseed oil paint
This paint is a white linseed oil paint specially designed for plastered surfaces. It is painted on pH-neutral plaster or on previously painted plaster. The paint contains gum turpentine and should not be diluted. Paint in two thin, even coats with a brush or roller. The paint dries to a semi-gloss finish.
