Outdoor finishing

Overview

Outdoors, a finish has two jobs: slow the damage from sunlight and keep liquid water out of the wood. The USDA Forest Products Laboratory's finishing chapter (Wood Handbook ch. 16) and its FinishLine sheets on outdoor furniture and decks are the basis for this entry. The main choice is penetrating (oil, water-repellent, semitransparent stain) versus film-forming (paint, solid-colour stain, varnish).

Pros

• Penetrating finishes are easy to renew: clean, then re-apply, with no stripping. The FPL recommends them over film finishes for decks.
• Film finishes last longest when intact. The Wood Handbook's service-life table gives about 10 to 20 years for paint on a suitable surface and 6 to 15 for solid-colour stain; against 1 to 3 years for a water-repellent preservative and 4 to 8 for semitransparent stain.
• Roof or shade protection extends every finish's life.

Cons

• Even the best clear exterior films last about 10 years fully exposed according to the Wood Handbook, and typical clear finishes last far less (table: tinted clear 2 to 3 years).
• The Wood Handbook rates film-forming finishes on decks as poor.
• Penetrating finishes need frequent renewal and, with natural oils, can feed mildew (FinishLine 008).
• A single week of south-facing sun on bare wood before painting is enough to shorten paint life: in an FPL test on western red cedar, 16 weeks of pre-weathering led to peeling within 3 years, one week to peeling after 13 years, and unweathered boards were fine after 20 years ("sunburn").

Suitability

Naturally durable species (Western red cedar, Coast redwood, Teak, White oak, Black locust, Ipe) can be left unfinished and will weather silver-grey; finishes are for colour retention. Oily tropical woods repel finishes: see Teak and oily wood outdoor care. Pressure-treated pine needs its own plan: Pressure-treated lumber finishing. Species with wide, prominent latewood bands, such as Southern yellow pine and Douglas fir, are prone to early paint failure (Wood Handbook). For boats, see Marine and boat finishing.

Coats and application

1. Start clean and dry. Weathered surfaces: clean (oxygen bleach is gentler than chlorine for mildew cleaning per the Wood Handbook) and let dry. FinishLine 013 cautions against mixing bleach and ammonia.
2. Prime or seal all faces and end grain (Finishing end grain). End grain absorbs water fastest.
3. Choose by exposure: penetrating oil or semitransparent stain on decks and rails; primer plus acrylic-latex topcoat or solid stain for siding and doors where you want a film; marine or spar varnish for clear film on door and furniture (Spar varnish, Waterborne acrylic and urethane-acrylic topcoats).
4. Apply by brush, working into the wood and wiping off excess penetrating oil.
5. Schedule: inspect each spring. Re-coat penetrating finishes when water stops beading, not on a calendar. Re-coat films before bare wood shows.

Compatibility

Film over an old film needs sanded, sound adhesion. Do not paint over creosote or oil-borne treated wood (see Exterior wood preservatives). Interior oils and wax finishes (Danish, hardwax) are not made for exterior exposure; check the maker's label. Fire-retardant-treated wood may darken and affect adhesion (Wood Handbook).

Care and repair

Wash gently; the Wood Handbook says power washing is less damaging to wood than sandblasting, particularly at low pressure. Spot-repair penetrating finishes by cleaning and re-coating. See Cleaning and care of finished furniture and Long-term finish ageing and patina.

Safety and environment

Fungicides in water-repellent finishes and semitransparent stains are toxic and may be herbicides; protect plants and soil (Wood Handbook). Food should not directly contact surfaces with mildewcide finishes (FinishLine 008). Oil rags self-heat: Oily rag safety.

Related

• Spar varnish, Linseed oil paint, Latex and acrylic enamel paint, Oil-based enamel paint
• Teak and oily wood outdoor care, Pressure-treated lumber finishing, Exterior wood preservatives
• Marine and boat finishing, Long-term finish ageing and patina
• Shou sugi ban (yakisugi): charred cladding
• Thermally modified wood, Kebony, Wood-plastic composite (WPC)