Outdoor wood furniture requires regular maintenance, and cleaning is only the first step. After cleaning, the addition of sealant or water repellent is vital. Topcoat products such as varnish or paint are not typically used on outdoor furniture, and cleaning is usually limited to the surface of the wood only. When the process is scheduled once or twice a year, the furniture should serve you for years to come.
Scrub and Wash
Never use a power washer to clean outdoor furniture. Power washers remove soft wood between grain lines and force water into pores. Save the power washer for stripping. Start with a mixture of dish soap and water in a bucket. Scrub the wood thoroughly with a brush to remove dirt, grime, mildew and mold. You don't need to be too careful with hardwoods such as teak or oak, since they are hard enough to resist scratching. If you're cleaning softer wood such as cedar or pine, don't scrub perpendicular across the grain. Scrub along grain lines only. If a soft brush doesn't do the trick, switch to a wire brush, but be careful because wire brushes can remove soft grain. Work on arms, legs or other parts individually to prevent cross-grain scratching.
Oxygen or Acid
If stains are too deep to scrub off using soap and water, mix powdered oxygen bleach with water. Apply the mixture to the wood with a brush. You should see a foaming action -- similar to how hydrogen peroxide foams on your skin -- when the bleach reacts with dirt, grime, mold or mildew. Allow it to remain on the wood for about 15 minutes and wash it off with a garden hose.
If you can't get stains out with oxygen bleach, they are likely the result of tannin or iron. Oxalic acid will do the job, but it's considered toxic. Follow instructions carefully, and don't mix the acid with other products. Redwood is in a category by itself. It may also require the use of oxalic acid because oxygen bleach reacts to redwood and darkens it. Use oxalic acid only if you're serious about cleaning stains on redwood.
Moisture and Grain
Drying is important. Mildew, mold and decay grow when moisture is present. Place the furniture in direct sunlight or in a warm, dry shed or garage to dry it. Don't be surprised to find that grain is more prominent after washing and drying. Water heightens the grain, and it's to be expected. Smooth the grain down by hand-sanding the furniture with medium-grit sandpaper. Palm the sandpaper and work on arms, legs and other parts individually, sanding and smoothing in the direction of the grain only.
Sealers and Oil
Apply a synthetic water repellent with a soft brush when the wood is dry. Resin-based, pigmented sealer/stain retains the original color of the furniture and works as a water-repellant. Match the color as needed. Apply the pigmented sealer/water-repellant to individual pieces, working in the direction of the grain to avoid overlapping strokes.
Clear sealers give wood the protection it needs but allow the furniture to turn a silver-gray color as it ages. Choose clear sealer for a rustic appearance.
The use of oil-based products has always been contentious for outdoor wood furniture. Sometimes manufacturers recommend oil-based products, but that oil can trap moisture, leading to mold. Check with the manufacturer before adding oil-based products to wood furniture.