Chromated Copper Arsenate is a pesticide commonly used in pressure treated wood. It protects the wood from deterioration, prolonging the life of outdoor wood structures such as decks and playgrounds
Pressure treated wood is protected from insect infestation, rot, and decay, but not from the damage caused by exposure to the sun, rain, and temperature changes. In fact, pressure treating makes the wood surface more porous and therefore more susceptible to moisture damage. Cycles of wet and dry, swelling and shrinking cause wood to warp, split, crack, become discoloured and unattractive - all of which leads to premature wood life and expensive repairs.
Recent studies have shown the possibility of rainwater leaching (releasing) CCA from treated wood. This can lead to CCA being present in the soil around a structure or as fine coating on the wood surface. There is still considerable debate about the effects of CCA residues as the amount of leaching occurring is so small.
People are sometimes concerned about leaching when CCA treated wood is used in playground equipment and decks, where leaching material gets onto children's hands and clothing and, with the tendency of young children to frequently put their hands into their mouths, can then be ingested by the child.
Coating is an efficient and economic way to minimise leaching from CCA treated timber and at the same time, protects the CCA treated timber from weathering.
Coatings that can limit the movement of water into and out of the treated timber will reduce the leaching of chromium, copper and arsenate. Paints, water repellents and stains can be used over CCA treated timber that is appropriately dry. Before painting, any loose CCA salt on the surface must be removed by sanding and hosing off. On very rare occasions, soluble CCA salts, if they have not been properly fixed in the wood can migrate through waterborne paints causing a cosmetic stain. In geothermal environments, staining through coatings can occur due to the reaction of copper with atmospheric gases.
When selecting a coating for finishing CCA treated timber there are a number of aesthetic choices. Aesthetic choice needs to be balanced against weathering performance and maintenance expectations. Paints perform best. Coatings that allow the wood grain to show through, such as clear finishes (or water repellents) and stains, allow ultraviolet rays through to the surface. These clears and stains have a shorter life than opaque paint systems and may be harder to clean.
Dark colours attract heat, which can put extra stress on the surface and lead to early paint failure. Pastel shades are recommended for best performance.
Resene provides high quality paints to coat CCA treated timbers - the life span of each product depend on surface preparation, application, and maintenance.
Resene Lumbersider satin acrylic
(Resene Sonyx 101 semi-gloss acrylic or Resene Hi-Glo gloss acrylic are also suitable topcoat options).
Resene Waterborne/Solventborne Woodsman
Resene Kwila Timber Stain
Regular maintenance must be undertaken once CCA timber has been coated with a paint or stain to ensure that the timber continues to be protected.
Resene recommend the following coating system (expected life 5-10 years):
Clean timber and remove sharp edges, then prime with one coat of Resene Wood Primer (solventborne) or Resene Quick Dry Acrylic Primer Undercoat (waterborne). Ideally all sides of timber framing and decking timber should be fully primed prior to erection of the deck.
Then apply a full coat of Resene Lumbersider (waterborne) followed by a coat of Resene Lumbersider modified with 500 gms of SRG Grit per litre. The latter coat should be applied at about 7-8 square metres per litre to produce a uniform skid resistant finish. An unmodified coat of Resene Lumbersider should then be applied at about 18 square metres per litre to give a washable finish.
This system of 1 coat of Resene Lumbersider modified with SRG Grit followed by an unmodified coat has been tested for Pedestrian Slip Resistance according to AS/NZS 3661.1:1993. The Building Code requires the coefficient of friction in access ways to be greater than 0.4 in wet conditions.
Tests carried out by Opus International Consultants Ltd, PO Box
30 845, Lower Hutt.
Test Report 99-527914.92. Mean coefficient of friction on wet surfaces 0.42.
If a stained finish is desired then two coats of Resene Kwila Timber Stain or Resene Solventborne Woodsman may be used. Stain finish systems will require a third coat after 1-2 summers. Note that these products have not been tested for slip resistance and will not conform to the Building Code in accessways.
Resene do not recommend clear finishes over exterior timber exposed to direct sunlight. Clears offer little UV protection of the timber surface. Early loss of adhesion will be caused due to UV breaking down the fibres on the timber surface.
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