What Is a Good Wood Preservative for a Fir?

Creosote

• Creosote is an Environmental Protection Agency (EPA)-registered, oily preservative once used commonly in the railroad industry. When combined with coal tar, it inhibits fungal growth and limits weathering due to moisture uptake in the wood. Creosote-treated fir is toxic to plants and releases unpleasant fumes, especially when the wood is newly treated and temperatures are high. While long-lasting, creosote-treated fir should not be used indoors or where it will come into frequent skin contact, and it is virtually impossible to paint or stain.

Pentachlorophenol

• Pentachlorophenol or PCP is another oil-type preservative regulated by the EPA. PCP must first be dissolved in a carrier solution such as mineral spirits or petroleum oil before being applied to fir or other wood. It is commonly used by the millwork industry and as a preservative for posts and poles. PCP treatment turns the wood light or dark brown depending on what is used as a carrier substance. It does not give off toxic fumes, but it is not recommended for applications where people will touch it frequently, such as handrails.

Chromated Copper Arsenate

• Chromated copper arsenate, or CCA, is a waterborne salt that binds tightly to wood fibers, making it relatively safe for plants that may come into contact with the lumber. It has either a greenish or brownish cast, depending on the process used, but unlike fir treated with oil-type preservatives, CCA-treated lumber can be painted and stained. New regulations in 2004 limit the use of CCA-treated lumber in residential applications like decks and fences.

Ammoniacal Alkaline Copper Quat

• Ammoniacal alkaline copper quat, or ACQ-B, is a recently developed wood preservative that protects fir against both fungal decay and insect damage without the use of arsenic. ACQ-B is more effective at penetrating Douglas fir than many other wood preservatives. Though it is not rated for marine use, it can be painted and stained and is more suitable for residential use than CCA or oil-type preservatives, making it an attractive choice.