information - Scottish Natural Heritage

Application methods
VACUUM PRESSURE
This is sometimes just referred to as pressure impregnation. The timber is
loaded into a cylinder which is sealed and a vacuum is drawn to partially
extract the air from the timber. The cylinder is then flooded with preservative
and pressure applied to force it into the wood. This type of treatment provides
the highest levels of protection for the most hazardous circumstances and is
used with either the CCA waterborne preservatives or creosote types.
DOUBLE VACUUM TREATMENT
This method again uses a cylinder into which the timber is loaded and an
initial vacuum drawn to remove the air; it is then flooded with preservative and
the vacuum released, causing the wood to suck in the preservative. This may
be followed by a holding period during which pressure may be applied before
a second vacuum is drawn to remove surplus solvents and to speed drying.
This method of application is only used with either the organic solvent type
preservatives or the pentachlorophenol in heavy oil, and it is the most
effective method of applying these types of preservative.
HOT AND COLD OPEN TANK
This method of application is only used to apply creosote and consists of
immersing the timber for a few hours in creosote heated to approximately
90° C. This causes expansion of the air within the timber. The timber is then
permitted to cool, still immersed in preservative, and the contraction
associated with the cooling draws in the preservative. This provides a useful
method of providing high levels of protection where creosote is an acceptable
preservative, and without the need for sophisticated equipment. Full details of
this procedure are provided by BRE/ PRL Technical Note 42, 'The Hot and
Cold Open Tank Process for Impregnating Timber.'
IMMERSION TREATMENTS
These simply require that the timber be totally submerged in preservative for a
defined period, which should not be less than three minutes. Only
preservatives of the organic solvent type or the fluid grades of creosote may
be applied by these means and the treatment is suitable for medium hazard
situations. This method requires no sophisticated equipment and a simple
temporary bath could be produced by laying two layers of heavy gauge
polythene over a brick or timber surround. Timbers may need to be weighted
or turned to ensure that all surfaces receive treatment. Particular attention
should be paid to manufacturers' safety warnings regarding storage, spillage
and operators' protection.
BRUSH OR HIGH PRESSURE SPRAY
These methods are generally not recognised as providing substantial
protection against decay due to the limited penetration which they achieve.
They are used only in situations where other methods cannot be employed.
Where no surface coatings are to be used reapplication (every 2 - 3 years)
must be anticipated. To obtain maximum benefit they should be applied as at
least two generous flood coats, paying particular attention to end grain and
joint areas, and the second coat should be applied as soon as the first coat
has started to dry on the surface, to keep the preservative moving into the
timber. When applied to relatively thin timbers, e.g., woven fence panels,
surface application can provide realistic protection against decay. Careful
attention should be paid to manufacturers' safety warnings and attention is
drawn to the particular hazards of splashes or spray entering the eyes.
page 14