Preserved wood products are typically cut and drilled during construction. Field treating these cuts and holes is critical to maintain the protective qualities of the wood.
During pressure treatment, preservatives are infused into wood cells to creates a protective shell. Cutting and drilling holes exposes parts of the wood that were not treated with preservatives, increasing the opportunities for decay fungi and insects to deteriorate the wood.
Building codes and American Wood Protection Association (AWPA) standards require field treating for preserved wood used in construction. Field treating cuts and holes in preserved wood enhances the overall durability and its benefits are far greater than the minor cost and time required.
Standards for Field Treating
Guidance for field treating is found in AWPA Standard M4, Standard for the Care of Preservative-Treated Wood Products. The standard notes "all cuts, holes, and injuries such as abrasions or holes from removal of nails and spikes which may penetrate the treated zone shall be field treated."
The most commonly available preservative for field treating wood in outdoor exposures is copper naphthenate. There are a variety of copper naphthenate-based preservatives available that contain the minimum 2% copper as recommended in the standard. AWPA Standards note that using copper naphthenate with 1% copper "is appropriate in those regions of the country where the higher concentration material is not readily available."
Preservatives with oxine copper also can be used for field treating. Oxine copper is typically used for treating larger wood products such as log homes, utility poles and guardrail posts, but is also suitable for decks. Oxine copper preservatives, containing the recommended minimum 0.12% copper metal, are available colorless or in various colors and have little odor.
Field treating is done using a brush to apply the preservative to the cut area or hole. Follow the preservative’s label for application instructions. When field treating wood placed over water, care should be taken to avoid dripping the preservative into the water.
The surface of the area treated should be cleaned before application, then coated liberally so the treatment can penetrate into the fiber. A standard paint brush can be used for most field treating. Be sure to coat all cuts, including the ends, and apply the preservative deep into the holes.
Preservatives for Field Treating
There are a number of topical preservatives available in the retail market designed for field treating preserved wood. These can be purchased at local home centers, building material retailers and paint stores who stock products for wood decks or online.
Some of the most popular copper naphthenate preservatives for field treating include:
Oxine copper preservatives available for field treating include: