There are two natural elements that will age and degrade wood in a hurry: ultraviolet light and water. Light ages wood prematurely, and water. whether from melting snow or tropical rain, causes wood to rot, joints to loosen and nails to pop. Using a wood preservative on exterior siding, decks, trim and even outdoor wood furniture significantly increases the lifespan and maintains the natural beauty of wood.
Anything that either coats or penetrates wood to protect it from the elements is a preservative by definition. Most commonly, clear or coloured stains and sealers are used on exterior wood, like decks, furniture and sometimes wood siding. Solid stains and paints are another option, and more commonly used on siding and trim. Any of these finishes can be either oil- or water-based.
Stain and sealers penetrates the wood fibres, essentially turning the outer layer of the wood into a protective coating. Oil-based stains contain petroleum based solvents which soak into the wood, while water-based stains contain silicone or stearate (wax) additives to repel water. UV protection is usually added with the pigments, although a small amount of UV protective additive is in the better clear stains. Typically, vertical surfaces like siding, deck edges and rails will last twice as long as the horizontal surfaces like windowsills and decking, because they are less exposed to full sun and standing water. Paints and solid stain preserve wood by forming a protective film over the surface.
Determine the usage and type of exposure the wood gets in order to choose the best wood preservative. While solid stain and paint are excellent choices for siding and trim, they may be a poor choice for decks. Coating a deck with solid or opaque stain is usually an irreversible step, since it is impractical and extremely time consuming to remove. They do not hold up as well under heavy foot traffic or in very wet conditions. Clear or coloured stain is the best choice for a deck preservative. While clear preservatives may have to be recoated every year and stains every two to four years, it is a fairly easy job. Shady and damp wood is susceptible to discolouration from mildew or fungus, so choosing a preservative with mildew and fungicides is important.
Local conditions may influence wood preservative choices. In very wet and humid climates, more frequent applications may be necessary and it's important to consider mildew protection. If indigenous waterproof woods like teak and redwood are used as building materials, they may not need as much attention as softer woods like cedar or pine. In hot, dry climates, ultraviolet protection will be more important than waterproofing. Generally, a tinted sealer or stain, or paint, provides better ultraviolet protection.
When applying any kind of clear preservative or stain that functions by soaking into the wood fibres, brushing the finish on provides the most long lasting finish by forcing the preservative deep into the wood.