Woodcare

INTRODUCTION

Understanding Timber

Wood is a natural material. It is formed as part of the life process of a tree whereby carbon dioxide and water are, by utilisation of energy from the sun, are converted into complex chemical substances – principally cellulose and lignin. These substances are laid down in the form of many joined boxes so that timber has a cellular structure. Perforations in the cell wall (pits) permit passage of sap, etc., from cell to cell.The trunk of the tree increases in size by adding successive layers of cells on to the outside of the wood already formed. The growth is not uniform throughout the year and this gives rise to annual rings, each consisting of a band of fast-grown early wood (Spring) and denser, slower-grown late wood (Summer). Softwood and Hardwood Timbers are commonly classed as softwoods and hardwoods. Softwoods are from coniferous trees which have needle-like leaves and are mostly evergreen. Examples of softwoods include Pine, Fir and Spruce.

Hardwoods are from broad-leaved trees like Oak, Ash, Elm, Beech and Chestnut. Tropical woods like Teak, Mahogany, Maple, Meranti, etc., are also hardwoods.

Softwoods are not always mechanically soft nor hardwoods always hard.

Moisture in Timber

Wood is porous and, in the living tree, the hollow cells are essentially filled with a watery fluid known as sap. Before timber can be used, much of this sap must be dried out – otherwise considerable dimensional changes would occur after it dries out.

Timber correctly dried and seasoned for use in furniture and buildings contains between 9% and 15% moisture. Timber used for carcassing sometimes contains up to 22% moisture.

Fungi require water to grow, so the moisture content in timber will determine whether fungi can survive and grow. Normally, dry seasoned timber is attacked by insects but is immune to fungal attack – fungi will only attack when the moisture content exceeds 20%.

Treatment of New Timber

It is essential that all surfaces are treated. The deeper the preservative penetrates the wood, the more effective it becomes. Thus, the effectiveness of a treatment depends on the method of application and on the composition of the preservative. Most preservatives are carried in an organic solvent which enables them to penetrate easily and quickly into the wood.

Properties of Wood

Timber is mostly used as an engineering material because of it strength. It is precisely this property that fungi and insects affect most the whole object of wood preservation is to prevent loss of this strength.Two properties of major importance to wood preservation are the natural durability and the ease with which preservatives are absorobed.Most softwoods are highly susceptible to both fungi and insects – Western Cedar and Pitch Pine are two exceptions, though. Hardwood vary considerably though in their susceptibility. Many, such as Oak, Teak, and Mahogany are very resistant (but not immune) to attack. Others like Ash and Beech are not nearly as resistant.The sapwood of almost all timbers is very much less durable than the heartwood but it is also usually more permeable and, therefore, easier to preserve.Most softwood and hardwoods are capable of taking up a satisfactory preservative treatment, even the depth of penetration into heartwood may be limited. In some woods (Teak, Pitch Pine, and some Oaks) the heartwood cannot be satisfactorily preserved by normal techniques.

Why Wood Decays

Wood is formed as part of a living plant and it can provide nourishment for other living things – mainly fungi and insects. In drawing nourishment, they destroy the wood substance so that the whole loses strength and form and then begins to decay. Under natural conditions, this activity by fungi and insects performs the function of destroying fallen and diseased wood. In virgin forests, the fallen decayed trunk of one tree may form the seed bed for a row of young trees.

Preventing Decay

An attack of dry rot, wet rot or woodworm can be prevented by using efficient wood preservers on new wood. This is much less costly and simpler than treating any such attacks afterwards.

When making extensions or repairs to buildings, or when erecting sheds, greenhouses, etc., preservation is simple and economical and will protect these timbers for a long time.

Timbers used outdoors are especially liable to attack and, often, the only reliable way to preserve them is when they are being installed..

Paint protects the surface from the weather but does not preserve against fungus attack.

Painting

BEST PRACTICE

There are two main classifications of timber, i.e. softwoods and hardwoods.
Softwoods come from coniferous trees (cone bearing) while hardwoods from deciduous trees, (broad leaved).
Softwoods are not durable when used externally, unless protected by a suitable wood preservative, followed by a paint, woodstain or varnish system.
Hardwoods are more durable generally and do not require preservative treatment. However, oily types do require pre treatment with solvent to remove oily residues.
Ensure surfaces are dry, clean and free from dirt, oil, wax, polish, etc before priming and painting.
Moisture content should not exceed 18%.
Double check all vulnerable areas of old paint for adhesion, i.e. lower rails, cills, etc., and completely remove coatings from these sections if condition is poor.
Check for any defective putty or beadings.
Allow new putty to form a hard skin before painting.
Use a suitable proprietary wood preservative on non durable, non pre-treated softwood.
Oily hardwoods should be wiped over with White Spirit to remove oily residues before priming.
Dispose of solvent soaked cloths in a lidded metal container.
Seal well all end grains.
Wet abrade where possible – it is more effective and creation of dry dust particles is reduced.
Use good quality exterior fillers outside.
Avoid painting in temperatures below 5° C or when rain is expected during application or drying periods.
Tightly re-seal lids and shake tin to form seal.

Surface

PREPARATION

All surfaces must be sound, clean and free from anything that will interfere with the adhesion of the material to be applied.
Prior to painting the moisture content should not exceed 18%.
Rub down new timber with fine abrasive paper to obtain a smooth finish. Lightly round off sharp edges to a 3mm radius. Dust off.
Remove all areas of detective paint coatings back to bare wood by scraping and /or use hot air paint stripper or proprietary paint remover as per manufacturer’s instructions.
Rub down exposed timber with abrasive paper to obtain a smooth finish.
Feather off edges. Dust off.
Remove excess resin from any live knots by use of a hot air paint stripper.
Wipe immediately with methylated spirit to remove residue.
Apply two coats of fresh shellac knotting to all exposed knots and resinous areas of timber.
To all non-durable, non-preservative treated bare softwood timber, brush apply a proprietary timber preservative to all exposed surfaces and end grains, in accordance with the manufacturer’s instructions.
Allow solvent to dry thoroughly before over-coating.
Scrub and swab oily hardwood surfaces with white spirit, frequently changing the face of cloths.
Allow solvent to evaporate before priming.
Rake out any defective putty fillets and cut out all rotted timber.
Replace rotted timber with new sound timber, pre-treated with suitable timber preservative, as already described.
Prime all bare timber. Including exposed putty rebates, with appropriate Albany Primer.
Wash all remaining areas of sound paintwork with a mild detergent solution to remove dirt or grease deposits.
Wet abrade surfaces with waterproof abrasive paper to form a key. Rinse thoroughly with clean water to remove all residues. Allow to dry.
Ensure all pin, nail and screw heads are sunk well below the surface and make good all surface imperfections with a suitable wood filler.
Allow filler to dry. Rub down smooth and level to existing surface. Dust off.
Remake putty fillets with linseed oil putty and allow putty to form a hard skin before painting.
Allow overnight drying between coats in the case of solvent borne paints. Lightly denib and dust off before over-coating.
An extra of Undercoat may be required to help fill surfaces on new work and also on re-paints where a contrast colour is involved.
Some strong colours may require extra coats.

Woodcare

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