BUILDING BRIEFING:

Surface finishes

- exterior joinery

Traditionally most joinery was of timber and this article concentrates on exterior surface finishes for timber joinery - windows, doors and other trim.Traditional and new technologies and their appropriate-ness are briefly explained. The importance of preparation is also outlined by Nigel Dann, Senior Lecturer in the Faculty of the Built Environment, University of the West of England, Bristol.

Wood

Wood is a natural material built up from microscopic tubes that feed food and water through the tree. The moisture content of wood must be reduced through a process called seasoning. This can be done through a natural drying process or by using a vacuum oven or 'kiln' to speed up the process. If the timber is to be stable and maintain its dimensions and appearance moisture content must be kept within limits. Fungal attack and insect attack is also moisture dependant.

The application of surface finishes, or decoration, has a number of purposes:

• To increase durability, largely through moisture and ultra violet light protection

• To maintain functionality, largely through avoidance of saturation

• To provide an aesthetic finish

Essentially there are two alternative commonly used forms of decoration: paints and stains.

Paints

Paints are most commonly used and provide opaque and potentially durable finishes. Paints consist of three parts:

Pigments: - which impart colour and opacity (useful in protecting from UV radiation and hiding previous colours). In the past naturally occurring and metal-based pigments were common; nowadays synthetic pigments offer both economy and reduced health risks

Binders: - are resins that are the film forming component of paint, binding the pigments together. In the past milk, bones and various oils were common; nowadays synthetic resins, such as vinyl, polyurethane and particularly alkyds, predominate.

Dilutant: - in which pigments and binders are suspended. This can be water or natural / man-made solvents (turpentine, white spirit, alcohol etc). Once applied the dilutants evaporate leaving the pigments and binders as a surface coating.

Alkyd gloss paints

Essentially, the glossier a paint the better its durability and weather resistance, and the higher the proportion of binder to pigment, the glossier the finish. The most commonly used glosses are solvent based alkyd resin paints. These require a series of coats to build up a suitable level of protection:-

An initial coat or primer to seal new or stripped timber against subsequent coats being absorbed into the timber. For example, aluminium primer is useful for resinous timbers. Oil-based solvent primers were common 20 years ago but are increasingly being replaced by fast drying acrylic water based ones. Some commentators suggest that these do not perform well externally.

A non-glossy undercoat gives both opacity and film-build to primed or previously painted timber. Alkyd undercoats have a tendency to brittleness and some believe that selecting multiple top coats is preferable to multiple undercoats. A common specification is: primer, undercoat and two top coats.

Advantages of alkyd gloss systems

• high opacity;

• good water and UV repellancy;

• a versatile colour range at reasonable cost.

Disadvantages

• are relatively complicated to apply - 3 coats and all the inter-coat preparation (see below);

• give off high levels of polluting Volatile Organic Compounds (VOCs) as they are solvent based;

• have a relatively frequent maintenance cycle (2-5 years);

• embrittle with age (particularly where significantly exposed to UV, e.g. on south facing facades for example); and, most significantly,

• are vulnerable to timber movement, resulting in cracking or flaking.

Technological advance

Timber is a hygroscopic material: it acts like a sponge, swelling and shrinking as moisture is absorbed or evaporates according to atmospheric conditions. Whilst alkyd paints attempt to provide a waterproof barrier for the joinery, there are several paths for moisture into the timber: ineffective putties, end grain timber and microscopic cracks in the paint film are most common.

Timber movement at joints has cracked the paint finish

Manufacturers have responded to this and developed micro porous paints. These are designed to shed water whilst increasing the vapour permeability of the paint film, allowing a greater proportion of any moisture getting behind the paint surface to evaporate out through the paint. The increased stability of the timber reduces the incidence of cracking and flaking of the paint film, a virtuous circle. Tests are favourable; some suggest that an extended 5-7 year maintenance cycle can be achieved, but the advantages of these micro porous gloss paints are reliant on adherence to application instructions. For example, the reduced number of coats, frequently one combined primer/undercoat and a single gloss topcoat, encourages porosity, but means that the quality of the painting must be exceptional.Changes in timber moisture content can cause problems with alkyd paints because as the wood moves, the rather brittle paint cracks allowing more moisture into the timber. As these paints are effective water barriers, part of this moisture becomes trapped behind the paint film, leading to a loss of paint adhesion, more cracks/flaking and a continuing cycle as more moisture is absorbed by the timber.  Some external gloss paints claiming to be effective after a single coat should be avoided, as there is not sufficient build in such paints to provide effective UV and water shedding. Increasingly new external gloss paints are water based as new acrylic binders have been developed.

Wood stains

A thick coat of paint on a poorly

prepared surface results in flaking

These remain less common in existing buildings but are relatively common in new houses. They are of two types:

‘Low solid’ wood stains: - which consist of a very thin paint with a very small proportion of binder. Here the pigment is
drawn into the surface of the timber and there is little surface build, leaving the grain pattern of the timber intact. These generally have very high porosity, but as there is little surface build they have reduced water shedding qualities.

‘High solid’ wood stains: - are significantly more popular and are, in effect, a form of translucent paint. They contain less pigment but, as they have a surface build, they offer better resistance to weather than low solid stains.  An advantage of such products is that generally a single product applied in two coats and can offer several years of service; 5 years is frequently claimed. Some of these high solid stains also claim to be micro porous.

Preparation, preparation & preparation

Appropriate preparation is the single most important activity in any external redecoration programme. The key issues are;

Minimising the moisture content of timber:

• Don’t leave timber exposed for long periods;

• Avoid painting during damp periods (October-April) or when it will rain before the paint being used will dry;

• Pay particular repair attention to areas which enable moisture ingress (ineffective glazing putty, - a frequent path for water ingress, exposed end grain, horizontal timbers that do not shed water etc.).

Fill and repair damaged timbers:

• Use high quality external fillers;

• Use resins to treat soft patches or, even better;

• Replace timber surfaces/sections/ whole components that require extensive repair. Ensure timber is well seasoned to avoid differential movement.

Follow manufacturers instructions:

• Stir paint thoroughly (where required to);

• Use the appropriate primers and undercoats;

• Make sure that you treat any exposed knots with suitable knotting compound.

Before applying products:

• Sand timber along the grain, and very lightly on sharp edges (paint will not stick to them) and always sand between coats to improves adhesion by providing a key for subsequent coats.

Strip existing paint from joinery only if:

• It has significant defects which make repainting on top of it difficult (cracking, flaking, blistering or rot);

• The new paint system about to be applied is not compatible i.e. there is little point in applying a micro porous paint on existing non-porous paintwork;

• You have checked that the existing paint is not lead based (essentially any pre-1965 existing paint);

• If you want to use a clear stain or varnish.

Avoid over-thick coats:

• For most traditional paint systems several thin coats are far more effective than fewer thick ones. Additionally, joinery may become impossible to operate if too much paint is applied. It is better to anticipate and allow for easing and adjustment of joinery before decoration, so that any exposed surfaces can be covered, rather than post-decoration when the new surface finish may be compromised.