12.06 Finishing timber

Categories: Finishes & Corrosion

Introduction
There are many species of timber which find their way into the facade industry, both softwoods and hardwoods. Whilst some timbers are particularly durable in their natural state, many are treated with preservatives and surface coatings to improve their durability. The use of green timber is increasing, and more new timbers are being introduced from tropical sources.
 

Main forms of deterioration
The main forms off deterioration are fungal decay and attack by wood boring insects. Weathering also causes degradation of wood.

Timber may be subject to various forms of fungal attack. Some, such as blue stain fungi, cause unsightly staining but do not affect the strength or durability of the wood. More commonly known forms of fungal attack, such as wet and dry rots, cause decay of the timber. The heartwood of some species is resistant to fungal attack but the sapwood of all species is vulnerable. Fungal attack can be prevented by keeping the moisture content below 20% and preservatives may be used to provide additional protection where this cannot be achieved.

The risk of insect attack is less predictable than for fungal attack. Generally only the sapwood is affected and painted timber is rarely attacked. Preservatives may be used to provide protection to unpainted timber.

Exposure to ultraviolet radiation leads to breakdown of the lignin which binds the wood fibres together and leads to a light grey appearance. Although this is a surface effect, the resulting surface is more susceptible to attack by moulds and fungi. Unprotected wood exposed to the weather will also be subject to rapid changes in moisture content leading to splits and cracks.
 

Life expectancy and maintenance requirements
Where timber remains dry it may last for over 100 years without maintenance. However there are examples of window joinery that have shown evidence of decay after 5 to 10 years.

BS 5589 gives guidance on preservative treatments for external timber intended to achieve design lives of 30 or 60 years and BS 5268 gives treatments for structural timber intended to give a design life of 60 years. These treatments assume that good practice will be used in the design and construction to reduce the risk of decay. The preservative treatments are primarily designed to give protection against fungal attack but some treatments will also protect against insect attack. Externally exposed timber will also require protection against UV light.

Apart from durability considerations, exposed timber will normally require regular treatment to restrict moisture movement and maintain appearance. Preservative and decorative treatments are described below.
 

Preservatives
There are three types of preservatives, tar oils such as creosote, water borne preservatives and organic solvent borne preservatives. Although it is effective, treatment with creosote produces a persistent odour and cannot normally be painted hence it is not normally used in facades other than sheds and similar structures.

Water borne preservatives are split into those which contain salts which react in the wood to become insoluble and those which remain soluble. The most common preservatives which form insoluble salts are copper/chrome/arsenic (CCA) preservatives. They provide high resistance to both insect and fungal attack and are suitable for external timber in high hazard locations. Treated timber is pale green and can be painted but can also be used without further protection. Preservatives based on boron compounds remain soluble. They are resistant to both insect and fungal attack but are only suitable for internal locations where leaching us unlikely to occur.

Solvent borne preservatives contain fungicide and/or insecticide in a hydrocarbon solvent. Fungicides include pentachlorophenol, zinc and copper napthenates and chlorinated napthalenes; insecticides include lindane and permethrin. Treated timber is suitable for low to medium hazard zones and can be painted. This form of treatment is commonly used for window frames.

Preservatives may be applied by brush, spray, immersion, steeping or pressure impregnation. Pressure impregnation, which involves immersing the timber in the preservative under pressure, gives the deepest penetration. The full treatment cycle may include vacuum stages before and after the pressure stage. The final vacuum stage being to remove excess preservative. The pressures used vary, CCA preservatives are usually applied using a high pressure process where the applied pressure may be up to 1400kPa whereas the solvent borne preservatives are applied using lower pressures. One commonly used treatment regime uses no positive pressure with vacuum stages before and after.

Preservatives are toxic chemicals which must be used in accordance with the safety regulations. There may also be hazards associated with the use and disposal of treated timber. Unnecessary use of preservatives is undesirable however application of preservatives is more effective and economical when carried out on the basic wood than as a remedial measure after construction. Control of waste and release of solvents can also be controlled more effectively under factory conditions.
 

Paints
Paints provide a distinct surface film with low permeability which protects timber by forming a barrier to ultra violet radiation and controlling the moisture content. Keeping the moisture content low minimises the risk of decay and insect attack and also reduces moisture movements which affect the operation of windows and doors.

Traditional paint systems are solvent based typically with an alkyd resin binder. Recently water borne acrylic based paints have been developed. The acrylic paints are relatively quick drying and can be recoated in 2-6 hours but are vulnerable to moisture and low temperatures during drying. The solvent based paints are more tolerant of poor conditions during application but require 12-24 hours between coats. Acrylic resin paints are also less tolerant of degraded wood surfaces. After application the acrylic paints have greater flexibility which improves durability. A maintenance interval of 5-8 years is quoted for acrylic paints compared to 3-4 years for traditional solvent borne paints although modern solvent paints developed specifically for exterior use may give an intermediate level of performance.

Whatever paint system is chosen the performance will be adversely affected by poor design and workmanship. Sealing the end grain before assembly of joinery, avoiding sharp arisses and providing good drainage of water all improve performance.

Traditional paint systems require a primer to seal the surface and provide adhesion for the main coats, an undercoat to provide opacity and a top coat to provide the decorative finish and overall protection. Modern paint systems may combine two of these functions in a single component which may require more than one coat.
 

Varnishes
Varnishes are clear surface coatings which are not generally recommended for exterior use as the resin binder is liable to embrittlement and the underlying wood is not protected from ultraviolet radiation. A maintenance interval of one year may be required to maintain the integrity of the coating.
 

Stains
Stains are pigmented resin solutions which often contain organic fungicides. The amount of penetration into the wood and thickness of surface coating vary from zero build stains which mainly penetrate and give a surface coating thickness less than 5mm to medium build stains which give little penetration and a coating thickness of 30-40mm. Low build stains give intermediate characteristics with a coating 10-15mm thick. The permeability of the coating decreases with thickness and only the medium build stains are recommended for joinery where control of moisture content, and hence movement, is important. Low build stains are suitable for cladding and zero build stains are normally limited to fencing and sheds. Stains provide protection against ultraviolet radiation by absorption by the pigment. Dark or highly pigmented stains provide better protection but darker colours will also lead to greater thermal movements. Fungicides incorporated in stains prevent mould growth on the surface but are not a substitute for preservative treatment of the wood.

Stains weather by erosion hence flaking is less likely than for paints but can occur with medium build stains. Recoating is generally easier than repainting but needs to be carried out before the coating has eroded sufficiently to allow degradation of the wood. A maintenance interval of up to 3 years is possible for low build stains and 3-5 years for medium build stains.