Remove staining, mildew and old finish with a commercial deck cleaning solution. Pressure washing is almost always a must on older decks, but be sure not to use too much pressure. You can damage the wood if it's turned up too high. Usually 1500-2500 PSI is adequate for pressure treated lumber. Stay between 1200-1500 PSI for cedar and redwood.

After cleaning the wood, let surface dry very well. Most coatings require that the wood should be dry before application. There are exceptions. Some newer sealers can be applied to damp lumber, so it's possible to clean and waterproof your deck in one day. Read the directions on the product you're using for temperature and weather guidelines, drying times, etc., and follow them!

Choose your finish. You can use an oil base or water base sealer or stain, solid or semi-transparent. The best method for applying the material is with a brush or a paint pad on a long handle, available at any hardware store. Most materials can be sprayed also, but you will end up needing a lot more product this way. For a large deck, spraying will save time but you should brush the material into the wood. This will help with uniformity and provide a better looking job.

There's a myth that wood should "weather" untreated for several seasons before being treated. This is untrue and leaves the deck vulnerable to damage. New pressure treated lumber should be allowed to weather for 30 days before being treated. Some products can be applied immediately to new pressure treated lumber -- check the label. Cedar and redwood can be treated immediately.

Test your deck by splashing water on it. If the water is absorbed into the wood it has little or no seal and will decay unless treated. If the water pools into beads or runs off the deck it is protected.

Solid color stain will help extend the life of an older deck that is starting to crack and check. Apply one coat of solid stain, let the deck weather for a year, or over the winter, and then apply another coat. Don't try building the material up too much at once.

If clear coating use 2 coats and apply a light coat after 6-8 months of weathering.

Seal the open end grains of the wood well. This will help stop moisture from getting into the lumber.

A roller is recommended over a sprayer. The latter will consume a lot more sealer and is difficult to control.

A little household bleach mixed 50-50 with water is great for killing mildew that may grow on the deck after you refinish. If your deck is getting some mildew or green moss, try a small spot to check for color changes. Use less bleach and more water to kill the mildew and not bleach out the color. Wear gloves, skin protection and eye protection when applying bleach.

When letting wood season, use New Wood Defender. If there are hairline cracks/gaps in the wood these should be filled. You can do this by applying oil then using a sanding block to run over the area with fine grade sanding paper. This gradually creates a wood/oil slury which fills the cracks and enhances the beauty of wood.

A deck that does not get a lot of sunlight is more likely to have mildew staining and green moss. This is a black staining or green fungus. Try to avoid oil based products if the deck doesn't get a good amount of sunlight.

Use a light color stain if your deck gets a lot of sunlight. A dark stain will make the wood get hotter.

A clear finish usually needs to be treated more often.

Do not use varnish!

Treated lumber does not hold paint well, so avoid painting.

If the deck is near a chlorine swimming pool, check your selection of stains and oils because the chlorine vapours can cause some oils to become opaque.

Be careful when using a sprayer and sealant on decks. The overspray may kill nearby grass and shrubs. To prevent this cover the adjacent greenery with a tarp or newspapers.

The sealant may drip between the decking boards, use tarps beneath the deck to avoid staining a cement pad. (2nd story deck)

FIRE HAZARD: Before disposing of rags, spread them out to dry completely. Heat generated from evaporating finish on a pile of rags can start a flash fire. If you are using latex stain, clean equipment with soapy water. If you are using oil-based sealers, clean with paint thinner or mineral spirits..