Exterior wood preservatives

Overview

A preservative stops decay fungi and insects; it is not a finish, it does not control UV, and it does not make wood look good. Industrial pressure treatment forces preservative deep into the wood, whereas a brushed-on product protects a thin layer. This entry sets out what the USDA Wood Handbook (chapter 15) and manufacturers say about the main types and what to do with older treated or reclaimed timber. It does not recommend any chemical for a specific use: check the product label, which is a legal document in the US.

Pros

• Deep factory treatment protects the whole section, which a surface finish does not.
• Borates diffuse into wet wood, so they are used as remedial treatment (Wood Handbook), and manufacturers such as Nisus describe them as low-toxicity (manufacturer claim).
• Copper naphthenate is dark green and weathers to light brown, and the Wood Handbook says it is not a restricted-use pesticide.

Cons

• Borates are highly leachable and suitable only for above-ground use where the wood is protected from wetting (Wood Handbook).
• Copper naphthenate should be handled as an industrial pesticide, and creosote has been a restricted-use pesticide since 1986, limited to pressure-treatment facilities (Wood Handbook). Creosote is oily, unpaintable, and its vapour photosensitises skin.
• All treated wood loses a small percentage of its active ingredients (Wood Handbook).
• Do not burn treated wood. The Wood Handbook says it is not federal hazardous waste but must not go in open fires, stoves or boilers.

Suitability

Residential waterborne preservatives are copper-based or organic; copper azole is used for decking (Wood Handbook). Wolman's page describes Wolman E as a copper azole system for decks, fences, picnic tables and similar uses, while Wolman AG is non-metallic, per Arxada. For naturally durable timber, no preservative is needed: Western red cedar, Black locust, White oak heartwood. Pine and spruce need treatment for ground contact: Southern yellow pine. Thermally modified and acetylated woods are chemically different alternatives: Thermally modified wood, Acetylated wood (Accoya).

Coats and application

General practice is to treat cut ends and drilled holes on treated lumber with a preservative the label recommends for field cuts. Borate-based remedial treatments are applied to existing wood by liquid spray and similar methods; they go on the wood and not into the soil (Nisus). After borate treatment, protect the wood from wetting. Do not use treated wood for food-contact surfaces such as boards or tabletops; this is conservative advice, and the product label governs.

Compatibility

FinishLine 013 and the Wood Handbook say waterborne-preservative treated wood can be finished once it is clean and dry. The Wood Handbook says wood treated with coal-tar creosote or other dark oily preservatives is not paintable except with specially formulated finishes such as two-component epoxy paints. See Pressure-treated lumber finishing. A penetrating finish applied regularly may reduce leaching from older CCA-treated wood (EPA).

Care and repair

Legacy CCA. Chromated copper arsenate was voluntarily withdrawn from homeowner products in December 2003, so decks, play sets and picnic tables from before 2004 can contain it. EPA does not require removal of existing CCA structures or soil. For older wood, EPA advises washing hands after contact, wearing goggles and a dust mask when sawing, never burning it, and checking state and local rules for disposal (usually ordinary trash for homeowners, per EPA).

Creosote-treated and reclaimed. Creosote-treated timber is oily and unpaintable, and the Wood Handbook describes its vapour as photosensitising to skin; this guide would not use it for indoor or play uses and found no source clearing it for them. Reclaimed lumber of unknown history: see Reclaimed lumber and Pallet wood.

Safety and environment

Handle preservatives as pesticides: gloves, eye protection and the label's respirator advice. Fungicides in water-repellent finishes are toxic and may be herbicides (Wood Handbook). Do not allow runoff to reach soil, plants or water. Ask your waste authority about leftover product (Finishing safety and disposal). This guide is US-only; other countries regulate these chemicals differently.

Related

• Pressure-treated lumber finishing, Outdoor finishing
• Fire-retardant treatments: another wood treatment with finishing consequences
• Teak and oily wood outdoor care
• Southern yellow pine, Western red cedar, Reclaimed lumber