Linseed oil paint on plaster

Previously, it was very common to use oil paint as an exterior paint. The term oil paint refers to, besides linseed oil colours, composition colour / tempera paints, which all also consist of linseed oil paint, which have been combined with watery glue bound. But the classic linseed oil paint was used as exterior paint.

Since the knowledge of oil paint as an exterior paint had been as good as lost, the Center for Bygningsbevaring Raadvad in Denmark (Center for the preservation of old buildings in Raadvad) prepared a guide on how to use linseed oil paint as exterior paint. This technique I still practiced today in Sweden, England, and the Netherlands regularly. The guide was created based on the experiences of these countries.

linseed oil inks are not alkali-resistant. That means the colour would be broken down by the base materials or even destroyed. Linseed oil colours "saponify" or “soap-like” and lose both strength and cohesiveness. Although this is also an advantage, aesthetically and structurally, this weathering process should not take place too quickly. Studies of old paint layers on historical buildings have shown that a new façade or exterior treatment was necessary approximately every 8-10 years. After the minimum of 8 years has passed the paint layered has been weathered enough and therefore easier to remove in order to prepare the surface for a new coat. Nevertheless, it is still important to effectively "isolate" the linseed oil and free it from base materials in the plaster with the use of a primer.

As with any paint treatment, the ground surface must be treated with linseed oil before it can be painted. Instructions for this procedure can be found in old technical manuals or even instructions. Many materials, such as white lead may no longer be used today because of their harmful effects.

For example, if the lime content in plaster is neutralized with fluosilicate, then use warm linseed oil to prime the masonry with warm linseed oil, which has been heavily diluted with turpentine prior to use would be extremely dangerous and harmful. When applied to such large surface areas, it would be extremely harmful to one’s health.

Today, it is much more viable and likely that one would work with environmentally sustainable products that are not harmful or health threatening. Therefore, only primers which do not contain Fluat, white lead or turpentine can be considered.

Smaller areas can be very well isolated with shellac. It is dense and free of toxins.

Another method would be to wait with the linseed oil paint, until the lime or other hydraulic lime plaster is completely carbonized.

Newer plasters can be checked with the chemical substance phenolphthalein to see whether they are carbonized. If phenolphthalein turns reddish purple when applied, then the plaster is not carbonized. It would be advised to wait before painting. Usually at least half a year by a new layer of plaster.

Linseed oil paint for outdoor use should consist of boiled linseed oil and pigment paste consisting of pigments, which have been well rubbed or processed on a three-roll mill. Linseed oil paint can, like no other material, penetrate well and deep into the pores of the treated surface. Linseed oil paints do not dry by evaporation or the like, but by a chemical process in which the linseed oil absorbs the oxygen from the air. During this oxidation process linseed oil colours increase in "volume" and "presses" itself formally into the cavities of the surface. This process allows linseed to adhere to the surface very well. If linseed oil paint is applied on too thick, this will result in a wrinkled surface under which the underlying linseed oil cannot cure or dry, as the oxygen can no longer reach the surface.

Painting the exterior or facade with linseed oil paints will make it appear shiny in the beginning. Over time, the gloss level decreases and the surface is matt and have a beautiful pastel like appearance. Since the linseed oil paint weathered slowly, the rain seems to the surface clean as it naturally removes dirt from the surface.

Before applying linseed oil paint to the exterior structure, the surface must be completely tied. New plaster work and new brickwork should be at least one year old. Newly plastered masonry at least half a year old before a linseed oil paint can be applied. The masonry must be dry and may not have any permanently damp areas.

Newly constructed masonry can only be painted with linseed oil paint when all calcareous materials are fully bonded. This should take on average approximately 1 - 1 1⁄2 years.

Older buildings with a new plaster layer must cure for 1 1⁄2 - 2 months, before they can be painted with linseed oil. The waiting time depends on the thickness of the plaster, the plaster material used as well as the time of year.

If older buildings still have an intact, old plaster, they can be readily painted with linseed oil paint. Linseed oil will not adhere on masonry that is permanently moist or on moist spots. Smaller plaster repairs should dry for 3-4 weeks.

The surface to be painted should be completely dry and not exceed 18% residual moisture at the time of repainting.