Towards the end of 1941 the Minister of Works established a series of committees to investigate and report on the problems likely to be faced in post-war building. ‘The Painting of Buildings Committee’ was convened in January 1946 by the Paint Research Association with the object of reviewing practice in paint manufacture and painting technique. Amongst other things it was charged with examining the essential needs of buildings during the first three years of peace and the technical properties of paint products.
A booklet was produced in 1944 and this was revised in August 1946.
Part 1 deals with the three main classes of paints – Oil Paints, Cellulose Paints and Water Paints – in a very clear and succinct manner. It also tells us about the expected life of paints. As a potted guide to what was available at the time it is excellent. This is a continuation of the three earlier posts which dealt with the first two categories, with Water Paints and with a look at the life of paints.
The normal process of decay on exposure to weather is due to the continual and varying action of changes temperature, moisture, and sunlight. The destructive action of these natural agencies is materially increased by the impurities present in the atmosphere of towns, particularly in industrial areas where chemical fumes are prevalent. Sea air also has a decided tendency to accelerate decay. The life of the paint, which depends on its resistance to these destructive agencies is influenced both by the composition of the medium and that of the pigments incorporated in it.
The manner in which the medium decays is of practical importance for re-painting. Some paints become hard and brittle on exposure to the weather and crack and peel in patches, making it difficult to obtain a good surface for re-painting without the removal of old paint. Others retain their elasticity and gradually decay from the surface, leaving the pigments adhering loosely in the form of powder, and the paint only needs rubbing down to provide a satisfactory surface for repainting. This form of decay, known as ‘chalking’, is therefore preferable, provided it only develops slowly, so that the paint retains its protective value for a reasonable time.
The pigment content of a paint also has a material influence on the manner of decay; for example, some pigments harden the film and promote cracking, whilst others promote chalking. The durability of paints is affected by many other factors which the manufacturer studies in adjusting the composition of both pigment and medium so as to ensure an adequate life to the paint. Five years is generally regarded as a reasonable life for a paint of good quality on exterior surfaces, but this is naturally influenced considerably by the conditions of exposure; for example, disintegration is more rapid on the southern aspects of a building than the northern, owing to the greater variations of temperature, etc.
Prior to the general introduction of titanium dioxide in house paints in the 1960s the white pigments most in use were lead carbonate and zinc oxide. They each had their advantages and disadvantages when used to formulate exterior paints. Paints based exclusively on lead carbonate tended to chalk on outside exposure – a problem understood for many years. An account of 1774 reveals
“The third year the gloss is gone – in the fourth if you rub the painting with your finger, it will come off like so much dust.”
Whilst zinc oxide provided a hard, durable and glossy paint film, a coating based on this alone will tend to split and crack. However a blend of the two produced a film that was glossy and hard enough but one that chalked gradually which meant that when it came to maintenance a wash and rub down was usually sufficient before repainting.
The introduction of titanium dioxide, especially the rutile form meant that a non-toxic bright white pigment with high opacity could be used as the sole white pigment in all types of paints. However this was not to happen for at least a decade.
From about 1920 there were experiments with antimony oxide (sold commercially under the brand name Timonox in exterior paints, although this did have a tendency to discolour in the presence of sulphur compounds in the atmosphere and yellowed.
Lithopone was another white pigment that saw use as an additive to paints after the 1870s. However, it too discoloured on exteriors and when it was employed it was mainly on interiors and either in undercoats or as the main component of water paints.
Paints protected from the action of the weather, as on interior surfaces, will normally remain in good condition for many years, but reference may be made to some causes of premature failure which affect paints on both exterior and interior surfaces. Moisture on or below the surface at the time of painting is the most prevalent of these; such moisture will inevitably find a means of escape and in doing so disrupt the paint film by the formation of blisters or flaking.
Another trouble to be guarded against is the action of alkaline substances – soda, potash and, in a lesser degree, lime. Such substances destroy the oil in the paint, combining with it in the presence of moisture to form soluble substances with the resulting destruction of the paint. This form of attack is known as “saponification”, and its practical application is indicated by the following examples:
a. Portland cement, lime plasters, and similar surfaces are notoriously difficult to decorate successfully with oil paint unless they are thoroughly dry and matured. This is due to the combined action of the moisture and alkaline salts contained in such surfaces when new.
b. The practice of using water containing soda, soft soap or other highly alkaline soaps for cleaning paintwork should be avoided, especially for enamel and high gloss paints, as the alkali will destroy the gloss by its action on the paint. Washing with warm (not hot) soap and water is usually all that is necessary for cleaning gloss paints. A soap of good quality, substantially free from alkali (e.g. curd soap), should be used if possible, and it should be well washed off with clean water after application.
c. When paint in bad condition has to be removed as a preliminary to re-painting the cheap paint-removers made with preparations of caustic soda should never be used. The soda is liable to be absorbed by the wood and attack the fresh paint. Incidentally, such preparations are injurious to the hands of the workmen. Many brands of non-alkaline paint-removers are available. These are made with powerful solvents which are not miscible with water and have a marked odour. They are thus readily distinguished from the alkaline pastes or solutions, which are substantially free from odour and mix readily with water.
The destructive effect of alkaline substances on paint is not limited to this action on the medium. Some of the pigments in general use are discoloured if paint containing them is used for the decoration of new cement, plasters and similar surfaces. Many pigments, notably the earth colours, are quite stable under such conditions. It is advisable to consult the paint manufacturer on the selection of paints suitable for the decoration of such surfaces. This is especially the case if green is required, as Brunswick green, a combination of the two pigments most subject to attack [Prussian Blue and Chrome Yellow], is extensively used in the preparation of green paints.
To be continued…
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