Deck wood needs protection from many elements, especially moisture. Moisture can easily damage the wood on your deck when there is a lot at one time, such as during rainstorms, or a consistent amount, such as in areas with high humidity levels. By treating your deck with the proper finish, you not only can protect it from moisture, but you can also protect it from other harmful elements, such as mildew and UV light from the sun.
Clean debris from the deck. Rinse it off using a high-power nozzle on your hose. Then, scrub the surface with a stiff brush. If the deck is very dirty, you can use a mixture of water and washing powder to clean the deck. Rinse it off when you're done.
Allow the deck to dry completely. In the meantime, pick out the finish you will use to treat the deck wood. The finish should be water repellent, contain UV protection and a mildewcide if moisture and humidity are common issues.
Brush on the finish using a large paint roller. Be sure to treat all of the deck wood. Use small paintbrushes in corners and on railings and stairs, if necessary.
Let the finish dry and apply another coat for more protection. This will also be necessary if you're using an opaque wood finish.
Remove debris in the cracks of your deck wood with a sharp object, such as a scoring tool or putty knife.