Application of preservatives
A variety of application methods is available and their use will depend on operational and economic considerations. Some methods are closely associated with specific compositions.
Preparation
Timber undergoing preservative treatment should be in suitable condition, notably:
- free from fungal and insect attack
- with clean surface (mud, water, outer bark removed) and free from paint, polish, varnish or other finishes
- seasoned with a moisture content normally in a range 25-30 per cent (except for boron diffusion treatment)
- all cutting, drilling, profiling and sanding operations are best carried out before preservation. if this is not possible newly-exposed surfaces should be treated with preservative after cutting, though this may represent a future point of weakness.
Brushing, spraying, dipping
Brush coating is the simplest but least effective method of applying preservative. Timber treated in this way with two or more coats generally needs to be retreated every two or three years. Spraying is no more effective since insufficient surface penetration may result. Dipping timber into an organic solvent, usually for a minimum of three minutes, depends on the absorption characteristics of the timber for its effectiveness. Better treatment requires longer immersion.
A special form of dipping is the open tank method. Here timber is immersed in preservative, usually tar oil, which is heated to 65-90ºC, is held in the oil for between 30 minutes and four hours, and is then allowed to cool. Heating causes the air in the timber's cells to expand and, as cooling begins, the preservative takes the place of the lost air.
Double Vacuum Treatment
Wood waiting for treatment in a double vacuum plant.This process is widely used in Britain for treating external joinery. It relies on the use of a sealed tank, into which a vacuum is drawn. The tank is then flooded with preservative under controlled conditions. A second vacuum stage is used to remove excess liquid.
Vacuum Pressure Process
The oldest of the application processes, invented by Bethel in 1838, is still used and is one of a number of variants which rely on the use of a retort loaded with timber and filled with preservative under hydraulic pressure. There are several variations (e.g. 'Bethel', Rueping, Lowry) which differ according to whether or not vacuum is first applied and to the pressure cycles used. Processes are also known by the terms 'open' and 'closed' cell.
Diffusion Treatment
There are a number of processes in which green wood is treated with a water-soluble boron compound (usually disodium octaborate, 'timborising') which gradually diffuses into the wood. This can be done locally by boring holes, by injection or by bandaging. The most common method is for freshly sawn timber to be treated and then stacked and covered for at least 30 days. Timber must not be allowed to dry out and will usually require a moisture content of more than 50 per cent.
Hot immersion and sap replacement techniques have also been developed.
Remedial and In-situ Treatment
It may be possible to halt or delay rot in joinery by pressure injection of preservatives. This service is offered bv some members of the Property Care Association. An alternative approach is to insert borax rods into predrilled holes which are then stopped with filler.
Safety Precautions
All wood preservatives must be approved before sale by the Health & Safety Executive under the Control of Pesticides Regulations 1986. Recommendations given on product labels and data sheets, including the wearing of protective clothing during application, must be followed.