Wood pressure-treated with waterborne preservatives may be painted or stained provided the moisture content after treating is reduced to at lease 20 percent. The colour imparted to the wood by the preservative may affect the final shade of any stains used.
Wood that has been treated with creosote, creosote solutions or oilborne preservatives cannot be stained successfully, or painted.
Water-repellent preservative finishes contain waxes, oils, resins, preservatives and, sometimes, pigments. The waxes, oils and resins make the wood surface repellent to water while the preservatives impart mildew and decay resistance to the wood. Pigments add colour and protect the wood surfaces from destruction by ultra-violet light. Finish life is related to the pigmentation and ranges from two to four years. One application of a preservative finish is usually sufficient. Maintenance is simple. The surface is cleaned and washed before a new coat of finish is applied.
Effect of Preservatives on Wood Strength
Acid salts can lessen the strength of wood if they are present in large concentrations. The concentrations used in preservative-treatments of wood are sufficiently small so that they do not affect the strength properties under normal use conditions.
However, engineering codes require that a slight reduction in engineering properties be made to account for loss of strength and stiffness due to incising of pressure-treated wood.
In preparing a project specification, it is advisable to have all wood pieces sized, drilled, notched and trimmed to finish dimensions, prior to pressure-treating. Should field fabrication be unavoidable. adequate field treatment of all exposed, surfaces must be performed, using an acceptable brush-on preservative, to restore the integrity of the preservative-treatment.
Handling Practices for Preservative-Treated Wood
- Wear gloves when handling preservative-treated wood to avoid direct skin contact.
- Avoid frequent or prolonged inhalation of sawdust by wearing a mask or by performing cutting operations outdoors.
- When power-sawing and machining, wear goggles to protect eyes from flying particles.
- Wash exposed areas thoroughly after skin contact, and before eating or drinking.
- Do not dispose of treated wood by burning.
- Treated wood used for patios, decks, and walkways should be free of surface preservative residues.