Exterior wood furniture won't withstand elements without care. Hardwoods such as oak, teak or eucalyptus dry out or absorb moisture, resulting in cracks and warps as wood expands and contracts. Weather-resistant woods such as cedar and cypress fare better than pine or fir. Darker furniture such as redwood tends to lighten with age, and light-colored woods like cedar turn gray. Proper care allows it to age with grace.
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Soap and Water
For light cleaning of wood furniture, dish soap and water should do the job. Mix it in a bucket, and use a brush or cloth and use the soapy water to clean off dirt. Finish by wiping it down with a soft cloth and allow it to dry. If the furniture is stained or has mildew or mold, stronger solutions may be needed.
Mix powdered oxygen bleach with warm water, and apply the solution to the wood with a soft brush. It will foam as the mixture attacks the dirt and mildew. Allow the solution remain on the wood for 10 or 15 minutes, then use the brush to scrub off the residue, working parallel with grain lines as much as possible. Use a garden hose to wash off the wood. Never use a pressure washer on wood furniture, as it can damage it. Don't use oxygen bleach if you're working with redwood. It reacts to redwood and darkens it. Use oxalic acid instead of oxygen bleach to clean redwood.
Allow the wood to dry thoroughly after cleaning. Place it outdoors in a warm well-ventilated area or indoors with fans. You might notice that grain lines are more prominent after the furniture dries because water raises grain. Use a medium-grit sandpaper -- 100-grit is about right -- to smooth and flatten grain back down, working parallel with the grain. Take it a step further if desired by sanding it again with 120- or even up to 220-grit paper for a smoother surface.
Smooth and Prep
After sanding it smooth, use a synthetic water repellent or sealer/stain that contains a sunscreen to block the damaging effects of ultraviolet light. Oil-based products that can trap moisture inside the wood are discouraged. Colored sealers containing pigment will retain the original look of the furniture. Clear sealers are also fine, but they tend to offer limited UV-resistance and allow the wood to turn a soft, silvery gray color. Apply the sealer with a soft brush or rag, working on individual pieces -- arms, legs -- one at a time to prevent overlapping strokes. Work with grain patterns, applying the sealer parallel with grain lines. Most sealers require multiple coats for maximum protection.
Decay and Storage
Decay and degradation of wood furniture is caused by moisture and sun exposure. Prevent future problems by keeping your furniture dry, and protect it from the sun with sealer and/or shade whenever possible. When you're not using it, store it indoors in a garage or shed. If that's not possible, invest in outdoor furniture covers of some type. They will protect it from the elements to a certain degree. It's recommended that preservative/sealer or water repellent be applied once or twice a year to aid longevity.