Wood Finishing and Decoration

FINISHING/DECORATION

Most of our wood products will arrive unfinished and you will need to consider your desired finish, ideally before you even place an order. Here are some tips, advice and things to consider;

You may be thinking about clear finishes to ‘show off’ the beauty of the timber and graining. This is fine for normal internal conditions (domestic rooms were moisture content is low and heating is between 16-21 degrees Celsius) but for products to be used externally or in rooms were temperatures and moisture content is high - carefully consideration is needed.

For more on clear finishes see below.

The type and species of timber you choose will also influence the decoration or coating you use. Some timbers take to certain finishes better than others. We have indicated some specific considerations for each timber see materials. Used externally some softwoods can be resinous and cause issues with the decoration process and on-going maintenance. See below further information about resin in timber.

Period of maintenance and time between re-coating is also often a determining factor when choosing a finish. This is often determined by the decorating product manufacturer guidelines. Generally oils, waxes, varnishes and other clear finishes require re-coating more often whereas stains, traditional primer, undercoat and top coat systems will last much longer. Water based products are easy to apply, dry quickly and easy to clean up afterwards. Traditionally these have been inferior to oil/solvent based products but with the advances in technology and product development water based products are a very effective solution.

Wood finishes some of your options;

We recommend the following products and have highlighted some points with may help you choose your coating product.

Oil for wood, wood stains, paint for wood. For more on wood treatments and wood preservatives please go to: Timber Treatment - An Overview

Although we recommend the above products - it is important that you read and understand the specific products and their intended uses from the manufacturers, to ensure you choose a product that is; right for you, your chosen timber and the intended use/location. All of the above have very helpful technical advice teams that can advise you further. Details of which will be on their websites.

Correct and extensive preparation and application of coatings strictly in accordance with the manufacturer’s guidelines is important to the longevity and maintenance of your products.

Further in depth information about the points highlighted above;

TIMBER DECORATION APPLICATION

It is recommended that, exterior wood finishes, decoration be commenced as soon as possible after delivery or installation (preferably within days). Prolonged exposure of bare timber or primary/base coats will affect the long-term performance of subsequent coats (see further notes below).

Wood Protection and Wood Care

ENSURE SURFACES AND EDGES ARE IN GOOD CONDITION BEFORE APPLYING FINISHING COATS - Additional sanding and preparation is likely to be required and the extent of this will depend on the desired finish.

If the primer or base coat stain has deteriorated it should be re-coated before further finishing coats are applied.

If the primed / base coated windows have been exposed on site, without further decoration for longer than 3 months, (either fitted in the building or in an open storage area) the primer / base coat must be sanded back and a fresh primer / base coat applied.

Finishing should be carried out in dry weather using good exterior quality materials in accordance with the manufacturer's instructions.

On external surfaces, the finish should be an exterior paint or a `high build' stain. (`Low build' types of stain should not be used on external surfaces.) Second coats or undercoats must be applied within 3 months of primed/base coated windows becoming exposed to the elements.

It should be noted that the use of dark coloured paint or stain finishes, particularly if located on the south or south west elevations of buildings, will result in high surface temperatures and can increase the risk of resin exudation through the finish (see further notes on resin).

Tongue and grooved boarding or boards used to cover large surface areas - we recommend that, before installation, the tongues and grooved (or abutting surfaces) are coated with full decoration process. This not only provides good protection but also ensures that if/when movement occurs there is no visual 'break' in the finish.

It is also important to coat the complete surfaces of your product to ensure adequate protection and equal surface exposure. For example if the back (unseen, unexposed surface) of a skirting board was left uncoated and the face was decorated this could lead to considerable distortion and movement as the moisture levels to the opposing surfaces differ. This can occur especially when fixed to new walls recently plastered (moisture content high - then 'drawn' onto the timber as internal heating is introduced). This a one example but as you imagine a similar process can occur in all situations - think of cladding exposed to the elements from the face side.

CLEAR FINISHES

It maybe your desire for external joinery to be left unpainted so that it develops a natural appearance. However the effects of the weather and sunlight can be harmful to timber damaging the surface and leading to undue movement.

Timber exposed to the weather and not protected by a suitable finish will, quite rapidly, bleach or go grey giving the timber a neglected appearance. Further exposure can lead to surface checking and splitting due to rapid changes in the moisture content of the timber at the surface, which can differ greatly to the moisture content at the core. Directly after rainfall the moisture content of the timber surface can exceed fibre saturation point of 25% to 30%. In dry conditions heat from the sun and the drying effects of the wind can rapidly dry the surface of the timber, while the core will dry more slowly. This cyclic action and the rapid changes at the surface can cause stresses to be induced in the timber, leading to splits and checks.

Colour changes of unprotected timber may be partly due to the leaching of water soluble extractives, which contribute to the colour of the timber, and partly to bleaching by Ultra Violet light (UV). For example, oak contains extractives known as tannins (see further info about Oak below) which are water soluble and are leached from the surface by rainwater. The bleaching of the timber is solely caused by the effect of sunlight on lignin, one of the chemicals in timber, which is photosensitive and degrades and changes colour on exposure to UV.

Movement of timber is due to changes in moisture content. Distortion can occur as tangential movement (in the direction of the growth rings) can be up to twice the amount of radial movement (across the growth rings) and because the grain in timber is rarely perfectly straight. Modern paints and finishes are classified by their ability to affect the amount of movement in timber. For joinery, finishing systems should be classified by the BS EN 927 series of standards as suitable for stable end-use. That is, the coating should be formulated to keep the movement of the timber to a minimum. The coatings protect the surface of the timber from UV and direct wetting from rain and they permit water vapour to pass through allowing the timber to change moisture content. The stability of the timber is achieved by making the coating less vapour permeable causing the changes in moisture content to be slow. Uncoated timber does not benefit by this slowing effect and subsequently a greater amount of movement can occur and this can be detrimental to the joinery.

To maintain the appearance of the timber and to limit problems due to surface degradation and movement, a suitable coating should always be applied. As a minimum, joinery should be primed or stained with a base coat and finishing coats should be applied as soon as possible.

RESIN EXUDATION IN JOINERY TIMBER

SOURCE OF RESIN

Resin occurs in timber either as a natural substance or as a result of injury to the tree during growth.

When occurring naturally it will contribute to the character of the wood. In these circumstances it may be in sufficient quantities to present a recurrent problem. Timbers with naturally occurring resin are used in situations which take advantage of the effect of the resin, e.g. where increased natural durability is needed.

The other form of resin is as a result of physical or natural injury to a tree. These can occur around knots which have not fully grown with the main bole of the tree. Occasionally physical injury can result in the build-up of resin pockets within the tree.

The timber we use in the production of general joinery is usually selected so as to be free of naturally occurring resin. However, resin produced as a result of injury to the growing tree can still be present.

Once converted the timber does not produce any new resin. During the seasoning, drying and selection process timber with obvious resin pockets is eliminated. When practical, joinery standards permit the cutting out and making good of resin pockets in joinery products.

RESIN APPEARANCE

However, even after this selection process, resin can still be present in joinery timber and may not manifest itself until sometime later.

There are various reasons for resin to unexpectedly appear at a later stage in timber products. Resin contains naturally produced oils. Over a period of time these evaporate and the resin hardens. If the timber is left to stand for a significant period of time all the resin in any piece of timber will harden completely. However, in modern joinery production it is not practical to hold stocks of timber for this length of time. The kiln drying process, used to reduce the moisture content of joinery timber, will help any resin which has access to the surface to be drawn out and this is eliminated at an early stage.

Softwood joinery which is to be used in external conditions is often preservative treated and usually with a solvent based organic preservative. This process is neither affected by the presence of resin nor affects the resin.

The problem of resin exudation usually manifests itself in the finished joinery product after the surface finish has been applied. It can also take a number of years to appear.

The machining process can expose fine routes to the resin pockets. Set in position certain parts of a joinery product can be warmed either by local heating sources or direct sunlight. Internally, controlled heating systems can raise the surface temperature of joinery just sufficiently to draw the resin to the surface. Externally, a combination of direct sunlight and dark finishing systems can also raise the temperature and cause the resin to flow.

The resin exuded can show itself on the surface in two ways. If a micro porous paint or stain has been used the resin may bleed through the finish. If a non-porous paint finish has been used the resin will lift the finish off the timber and expose the surface below. In either case the timber can be left until the movement of resin has ceased. However, this is not a very satisfactory solution if the timber is subject to attack by the weather or the aesthetics of the joinery is destroyed.

REMOVING RESIN

If a micro porous paint or stain has been used the best solution is to leave the resin to harden on the surface. This can then be removed by light scraping. During the period of resin movement the finishing system will continue to perform its function and will only need redecorating once the hardened resin has been removed.

Where the finishing system has been lifted a different solution must be employed. It is preferable to leave the resin, as before, until it has hardened, then scrape off and redecorate using a full finishing system on the bare wood areas. However, this method can only be used where the timber will not be subject to the weather. Protection to the exposed wood must be carried out as soon as is practical to prevent the moisture content rising and further finishing coats lifting due to the presence of excess moisture.

It may be possible to induce more resin to the surface by cleaning off the existing resin and applying gentle heat to the surface of the timber with an electric type blow lamp. Gently wiping the surface using a lint free cloth moistened with methylated spirits will remove the surface resin. For micro porous finishes this process, if carried out carefully, will avoid the need to redecorate.

It is impossible to guarantee that resin will not appear and damage the decoration on any joinery item. In the past shellac and self-knotting primers have been used but these only claim to reduce, not eliminate, staining of the coating system. These can never be completely successful because modern coating systems are designed to be flexible. Either they will not adhere to the area of knotting or the resin will force the decoration to separate from the timber.

Once the resin has come out to the surface and the procedures above carried out it is unlikely that the problem will reappear.

CARE OF EXTERIOR OAK ITEMS

SPECIFIC CONSIDERATION NEEDED

Oak is not as resistant to weathering as oily hardwoods and, therefore, needs better protection when used outdoors. The open pores provide a route for water-ingress. Blue-stain mould spores are likely to be present on any piece of timber and will be activated by moisture. Another consideration with Oak is the high tannin content, if not properly sealed water (rain) can wash the tannin out of the wood leaving a patchy colour and staining to the surrounding areas (bricks, patio, etc.).

It is therefore essential to use a coating that will give a high-build, seal the pores effectively and so provide a barrier to water. The items should not be exposed without adequate protection. Clear treatments will not provide protection from UV rays (but will obviously allow the beauty of the Oak to be displayed).