Pressure-treated lumber finishing

Overview

Pressure-treated lumber is wood impregnated with preservative, and it often arrives wet. The usual mistake is to coat it too soon. The USDA Forest Products Laboratory (Wood Handbook ch. 16) says nothing coats wet wood well, and that treated lumber may still be above the fibre-saturation point on delivery; you can tell because it is exceptionally heavy or because water runs out when a small piece is squeezed in a vice or struck with a hammer. This entry gives the Forest Products Laboratory's guidance and notes where manufacturers' advice may differ.

When to use

For decks, rails, fences and garden structures built from waterborne-preservative-treated lumber (copper-based or organic; see Exterior wood preservatives). Not for creosote or oil-borne treated wood, which is oily and not paintable except with specially formulated finishes such as two-component epoxy paints (Wood Handbook). Fire-retardant-treated wood is different again: Fire-retardant treatments.

Materials and tools

• Deck cleaner or oxygen bleach, a stiff brush and a hose
• A penetrating finish: a water-repellent preservative or a semitransparent stain (FinishLine 013)
• A moisture meter, or the water-bead test (below)

Steps

1. Check how wet it is. Squeeze or strike a small offcut: water means too wet. If it is, wait until it dries and use a penetrating, not film-forming, finish (Wood Handbook). Alternatively buy lumber kiln-dried after treatment ("KDAT").
2. Wait. The Wood Handbook says wood treated with waterborne preservatives can be painted or stained if clean and dry, and that about one week should be enough for the surface to dry enough to finish. Preservative can bleed through latex paint or solid-colour stain if wood is still wet from treatment. FinishLine 013 gives a different, longer figure for decks with a factory-applied water repellent or a recent water-repellent preservative: weather 2 to 3 months before applying a semitransparent stain, and note that waiting up to a year opens the wood further but also brings checking. Manufacturers' own waiting periods vary; read the tag or the treater's guidance, which I could not find to cite here.
3. Test for absorption. The Wood Handbook says that if water beads on a surface, finishes are likely to have trouble sticking, and a few passes with sandpaper often cure it. Sprinkle water: if it soaks in, go ahead; if it beads, sand lightly or wait.
4. Clean. Scrub with a deck cleaner or oxygen-bleach solution; do not mix bleach with ammonia (FinishLine 013). Let it dry.
5. Apply a penetrating finish. FinishLine 013 says a water-repellent preservative has a service life of about one year on exposed deck surfaces but is the easiest to reapply, and penetrates end grain well; semitransparent stains protect pigment and mildewcide from sunlight and do not form a film that can peel. Seal cut ends (Finishing end grain).
6. Plan to re-coat when water stops beading. Most decks are retired because of checking and splitting, which timely finishing can limit (FinishLine 013).

Common problems

• Finish peels or blotches. Applied on wet wood or over a film that cannot bond. Strip loose finish and use a penetrating product.
• Stain does not soak in. A factory water repellent is blocking it; weather the wood as in step 2.
• Wood checks and splits as it dries. FinishLine 013 says decks are mostly retired because of checking and splitting, not decay, and that timely finishing could avoid many of the cracks.
• Metal staining or corrosion. The sources I opened do not address fastener compatibility for treated lumber; check the connector and treater's instructions. The Wood Handbook notes that fire-retardant treated wood can be corrosive to fasteners.
• Mildew. Water repellents without mildewcide often blotch (FinishLine 013); clean and treat.

Safety and environment

Older pre-2004 treated wood may contain CCA: wash hands after handling, wear goggles and a dust mask when sawing, never burn it, and follow local disposal rules (EPA). The Wood Handbook says treated wood must not be burned in open fires, stoves or boilers, which includes offcuts used as kindling. Mildewcides and fungicides in finishes are toxic and may be herbicides, so protect plants and soil.

Related

• Exterior wood preservatives, Outdoor finishing
• Finishing end grain, Wood moisture and acclimation before finishing
• Southern yellow pine: the usual treated species
• Finishing safety and disposal