Paint, Wood and Weather

Colin Mitchell-Rose

Wood is one of the oldest and most versatile building materials used by man. It is incredibly strong and can easily be shaped or carved and joined into elaborate structures. At the same time, it can also be very flexible: it can move with the wind and even absorb earth tremors. Wood has a great natural beauty which can be enhanced by polishing and varnishing; it can be sliced into thin layers of veneer that conceal humbler (and cheaper) types of wood. It is easily obtainable and can be harvested and modelled using the most simple of tools. And to modern man it has the important advantage of being a renewable resource and easily recyclable: a truly 'green' building material.

But wood does have one fault; it can rot. If water penetration raises the moisture content above a critical level (around 20 per cent), the timber can be attacked by various moulds and fungi and start to rot: it is, after all, biodegradable. There are also many types of beetles and their grubs that can weaken and destroy the integrity of wood if its moisture levels are suitable. Furthermore, wood will also gradually degrade when it is exposed to sunlight. This is because one of its main components, cellulose, undergoes auto-oxidation in the presence of UV radiation in sunlight, leading to surface bleaching. (Cellulose itself does not absorb UV, but lignin, hemi-celluloses and some dyes and pigments cause cellulose to deteriorate because they act as photo-sensitisers, absorbing UV radiation and transferring the energy. As a result some of the long molecular chains of cellulose break up, weakening the material.) As this only occurs at the surface it generally does not affect the structural integrity of the timber (see 'The Effects of Daylight' by Rebecca Ellison in The Building Conservation Directory 2000).

There are many ways of dealing with these challenges, but this article will only deal with the role that paint (in the widest sense) has in preserving exterior woodwork.

EARLY TREATMENTS

If wood is allowed to dry out and 'season' properly before use, it gains a marked degree of protection against rain and the elements, particularly if it does not remain damp for long. Seasoned oak, for example, will weather to an attractive silver-grey appearance and, in the right conditions, will remain durable without being painted.

One of the earliest ways of protecting the exposed timber frame of a building from the elements was to coat it with the same material as that used on the panels between, usually a limewash or colourwash. Limewash is a mixture of fat (non-hydraulic) lime in water, usually with a binder such as tallow. It was often coloured with earth pigments such as ochre, when it is sometimes referred to as a colourwash. However, the term 'colourwash' is also used to describe a blend of earth pigments such as ochre, bound with animal glue size and stale beer (see 'Colour Washing and Pencilling of Historic English Brickwork' by Gerard Lynch in the Journal of Architectural Conservation, July 2006). While neither of these would last long on timber, applying them regularly would give some protection to the wood and help fill any cracks or gaps between the timber frame and the panels. Often the timber was either left untreated or the limewash was partially brushed off, leaving the grain of the timber exposed.

Later it became fashionable to treat the exposed timber frame with liquids from coal tar deposits, the forerunner of creosote. This black and white effect was noted (and deplored) by a traveller to the Welsh Marches in 1790. It later spread widely through Victorian England and became known in the 1920s as 'Stockbroker Tudor'.

OIL AND STAINS

Some hardwoods, such as oak and chestnut, are naturally more durable than others, while some softwoods, such as western red cedar and larch, contain high levels of oils which act as a defence, so they weather to a grey appearance without harm. Wood treatments like teak oil can be used to replace the oil lost from the wood through weathering and so help to retain the original colour. Other materials like wax or fungicides can be added, as in the 'Madison formula' which was widely used on cedar cladding and other timber from the early 1950s. Creosote, which was also found to have excellent preservative and water-repelling properties, was a mixture of substances obtained from the distillation of coal tar, and was widely used industrially for many years for preserving fences, railway sleepers and telegraph poles. Unfortunately these properties made it dangerous to handle and creosote has recently been listed as a carcinogen and its non-industrial use banned. There are modern 'safe' alternatives that trade upon its name.

Wood stain is, in essence, a modern development of timber oil. The difference between stains and paints or varnishes is that stains are applied in thin layers that soak into the wood, rather than forming a film on the surface. There are many types of stain available but they usually contain a thin oil or resin that is absorbed by the wood, a water repellent such as silicone or wax, pigmentation to keep out UV radiation and a biocide to retard timber decay. Most stains have good 'microporosity', meaning that they allow the wood below to 'breathe' by letting water vapour escape through them.

VARNISHES

If a hard natural resin such as copal or rosin is boiled up with linseed oil, the result is a varnish. This has several cosmetic advantages over plain oil as it dries to a clear, hard, glossy finish which enhances the appearance of the wood, prevents water soaking into it and protects the surface from damage. However, there is still one important drawback: UV radiation will pass through the clear film and attack the surface of the wood. This causes the varnish film to loose adhesion and it soon starts to crack and flake, leaving the grey 'dead' wood exposed. If pigments are added to the varnish they will reflect some of the radiation, but at the cost of changing the appearance of the wood: after several applications the wood can appear a dark muddy brown. Recently it has been found that very fine particles can reflect the radiation without affecting the transparency of the varnish. These are called UV absorbers and are used in better-quality exterior varnishes, marketed as yacht, spar or marine varnish.

Of course, the best way to prevent UV attack of wood is to add enough pigment to make the varnish completely opaque; in other words, a paint. This forms a hard opaque film that should adhere firmly to the wood and also move and flex with it. It should have a certain degree of microporosity to allow water vapour to move in and out of the wood, while preventing excess moisture being absorbed into the wood.

WHITE LEAD PAINTS

It was a happy coincidence that when the most widely available white pigment, white lead (lead carbonate), was added to the most widely available drying oil, linseed oil, the two dried together to form a wonderfully flexible adhesive film. This is because white lead reacts with the linseed oil to form very flexible compounds which, chemically, are known as 'soaps'. These made paint based on white lead and linseed oil (usually with small additions of turpentine and driers) extremely successful for coating and protecting exterior timber. Rather than becoming brittle and cracking, the paint would gradually weather or 'chalk' away to a matt surface which could be easily recoated after a few years.

However, these paints dried slowly and developing tastes also demanded shinier paints. One solution was to use a varnish rather than just linseed oil as the binder for the paint. The so-called 'enamel' paints which this produced had a harder, glossy finish, but as they aged the chemical reactions which made them dry in the first place continued, making these paints brittle. When the wood flexed, the film cracked and flaked exposing the wood to the elements. Many modern alkyd resin-based gloss paints suffer from the same problems: great initial gloss but poor long-term protection.

Whilst the wood here has initially been protected from UV radiation by the opaque paint film, the modern gloss paint has become too brittle to cope with the slight movement of the wood and has cracked along the lines of the wood grain. The resulting curling back of the paint, caused by tension in the paint film, exposes the wood to UV radiation and loss of adhesion and flaking soon follows.

UV radiation has penetrated the clear varnish and attacked the surface of this wood, causing it to go grey. The resulting loss of adhesion has led to the widespread flaking of the varnish coat.

Although white lead was a very useful white pigment, it was also long known to be toxic, causing long-term chronic illnesses. Many attempts to find alternatives were made, not always with success, and by the mid 19th century zinc oxide was being widely used in white paints. However, for exterior paints it was less durable than white lead because, when it dried, its paint film was less flexible. A compromise which became very popular was a lead/zinc blend, as the zinc oxide was noticeably whiter than white lead but the film still retained good flexibility.

Titanium dioxide, a very white and non-toxic white pigment became available in the 1930s. The early version was based on anatase (one of three mineral forms of titanium dioxide) and was not ideal for outside use because it was very prone to chalking. Later versions based on the rutile form of the mineral were very much better and are now the industry standard for white paints.

While white pigments tended to grab the attention of chemists, other pigments were being developed. The traditional earth colours, ochre, umber and sienna, gave good coverage and hence protection to timber, and were also light-fast. Many brighter inorganic pigments were developed in the 18th and 19th centuries, but they were based on heavy metals such as lead, mercury, arsenic and copper and so had similar toxicity problems to white lead. The late 19th century saw the development of synthetic pigments and dyes made from petrochemicals, but early versions tended to fade in sunlight and so lose their UV protection. Today, most pigments used for tinting or colouring paints are synthetic organic pigments but have much better light fastness.

MODERN ALTERNATIVES

With so many 'traditional' paints or their ingredients no longer available today, what materials can a specifier use for conservation work? White lead paints are still available, but only for listed buildings of Grades I and II* in England and Wales, and grade A in Scotland, and formal approval has to be sought from English Heritage, Cadw or Historic Scotland. There are only a few suppliers left and the major UK paint makers are exerting pressure on government to rescind the present derogation for the use of white lead paints on historic buildings. It is very much a case of 'use it or lose it' and the outstanding properties, appearance and historical authenticity of white lead paints still make them first choice where they can be safely used.

For lead-free alternatives, several types of linseed-oil paints are available (most imported from Scandinavia) which have a long history of successful use in timber preservation. It is most important that the linseed oils used in the paint are suitably treated to avoid being attacked by mildew. Whilst the application of these paints can be time-consuming, particularly when older layers of 'modern' paints need to be removed, they will give an appreciably longer life, as well as weathering to an attractive appearance.

Another modern alternative is to use a water-borne acrylic paint. These paints can be glossy or matt and, being water based, release no harmful solvents. (Volatile organic compounds, or 'VOCs', can contribute to atmospheric pollution.) Other advantages include their non-yellowing, quick-drying properties, ease of application, gloss retention and most importantly, no loss of flexibility with aging. With such an impressive list of attributes, you might well wonder why they are not more widely used. This may well be due to suspicion of any new material, although they first appeared on the market over 35 years ago. The early examples performed extremely well, and there have been many technical improvements since then, but like any advanced paint, they perform best when applied to a clean surface, free from old failing paint layers, and this is not always possible. Water-borne acrylic paints last extremely well outside: they have even been used for painting television masts in such exposed locations as mountain tops with great success.

GENERAL PRINCIPLES

The first step in the choice of a paint system for protecting external timber is to decide what the final appearance of the timber should be, and then to select the appropriate coating: oil, stain, varnish, semi-transparent, opaque, matt or glossy. Whichever is chosen, there are a few principles that must be followed to achieve the best possible solution and avoid premature failure:

• New wood should be of the best available quality and should have been allowed to season properly
• Old wood must be sound, and any rot or decay treated before painting
• Preparation is the key to success: seek advice from the coating supplier as to what is required and ensure your contractor carries it out
• Whatever coating you choose, make sure that it will remain flexible and has adequate microporosity
• Establish a regular inspection and maintenance regime: a 'stitch in time' will save heavy repair costs later.