Most lumbers used for outdoor furniture naturally resist decay and withstand the elements. Although sealants help preserve color and protect against wear and tear, they aren't necessary. Left unfinished, the wood acquires a silvery-gray patina and requires little maintenance.
To remove stains or mildew, mix 1 gallon hot water with powdered oxygen bleach according to directions. Scrub the area with a soft-bristle brush, and rinse. To remove rust stains or bird droppings, or to restore original color, sand lightly along the grain using fine-grit sandpaper, and rinse. If the area is too large to sand, mix hot water with oxalic acid crystals (also called wood bleach or wood brightener) according to directions. Apply with a soft-bristle brush, and rinse. Don't use chlorine bleach, which can damage wood fibers.
To protect from ultraviolet rays, dirt, and moisture, or to preserve restored color, seal the wood. First, clean surfaces, and sand off any existing finishes. Touch up with paint if needed. Apply a clear water-repellent preservative, which contains a mildewcide, or a penetrating semitransparent stain. (The pigment in the stain helps minimize sun damage.) Consult the manufacturer's instructions to learn how often to reapply; in general, you'll need to do so every 1 to 3 years.