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Artist's Palette

Feb 2005

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Can I use an ordinary house paint as an archivable undercoat? - I gesso

Controversy certainly exists in the area of artists wanting to use archivable products in their work as well as being attracted to the economies of mass produced house paints. A major hurdle is immediately imposed by the fine art suppliers, damning such products as 'ordinary house paint'. Whilst lower quality products do exist, high quality, modern architectural coatings are, in fact, quite extraordinary products.

The principal ingredients in all paints are binders (which form the films), prime pigments (which give the colour and opacity) and extender pigments (which contribute to build, porosity and surface roughness). Although there can be interaction between some reactive pigments and extenders with the binder, durability of an undercoat is predominantly the domain of the binder.

Binders used in modern architectural coatings consist of polymers of very high molecular weight (read stable) with little to no reactive groups (read stable again). Major polymers used are P.V.A's, styrene acrylics and pure acrylics. P.V.A's tend to be somewhat water and alkali sensitive; styrene acrylics are slightly prone to yellowing (but nothing like linseed oil) and acrylics are pretty well trouble-free.

These polymers are not single entities but a series of materials ranging from soft, highly flexible binders through to hard and brittle. Paint manufacturers select polymers on the basis of the job it needs to do. For example exterior timber coatings require a much more flexible binder than does a concrete coating.

In undercoats, the amount of pigment and extender that is combined with the binder has a powerful effect on the final properties of the film. For the same binder, increasing amounts of pigments and extenders lead to easily sanded films with high levels of porosity. This porosity can be very pleasant to work on with water colours where penetration can occur even with typically non-staining colours. Highly pigmented undercoats will generally be less flexible and more likely to damage if a finished canvas were tightly rolled.

As one increases the level of binder, the porosity drops off and the undercoat becomes more suitable for use under acrylics. Flexibility and toughness increases. Further increases in binder produces even tougher films which are highly suited for use under oil-based paints.

A word about surface characteristics and adhesion! It is generally intuitively felt that smooth surfaces must be more difficult to adhere to than rough ones. In the case of acrylic (including styrene acrylic and P.V.A) undercoats, this is not the case. The surface nature of these polymers is such that they are extremely receptive to overcoating.

This is not to say that the surface characteristics are unimportant; they are indeed important but not for reasons of adhesion. Very smooth surfaces can allow a loaded paint brush to skid across the surface impacting upon the artist's control. Too coarse a surface may demand too much paint, also impacting on control. The ideal surface is uniform with a very slight roughness, which presents a surface that accepts paint in a reasonably predictable manner.

Modern architectural undercoats may not be traditional (but then when was gesso last made from rabbit's skin oil?) but they do offer sound, stable, archivable bases for the artist. It is difficult to imagine them degrading in a benign interior environment.

This particular article has been somewhat 'broad brush' and we realise that it may well raise questions - questions that we would love to respond to. So if you have any questions or suggestions of further information you would like to see, please let us know.


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