Whether you use paint, stain, or sealer, your deck will last longer and look better if you understand the plusses and minuses of each option.
The appearance and durability of a deck are affected by the finish you apply. Treated, rot-resistant, and synthetic deck materials don't require a finish, but a sealer, whether clear or colored, will extend the life of the wood.
The choice of a clear sealer or a colored stain is an aesthetic one. A clear sealer will preserve the natural coloration of redwood, cedar, and cypress. Because pressure-treated lumber usually has a greenish tint due to the chemical used in treating it, a clear sealer may not be desirable. These sealers are best applied with a brush but can also be applied with a sprayer. The advantage of a brush is that it tends to make better contact with the wood, filling any pores. A clear sealer protects against wear and tear on the wood and will prevent some scuff marks.
Redwood and cedar are rarely stained, because they have such distinctive colors. Cypress and pressure-treated wood, with their lighter colors, can be stained to match your setting or house. Stain ranges from the palest sand color to the reddish orange of redwood. Stains are also applied with a brush or sprayer. Because stains are more fluid than paint, you can use stain-soaked cloth to rub it on the wood. This is helpful when staining thin members such as railings.
Paint is best reserved for railings or other areas that won't be subject to a lot of wear. If you paint the decking itself, plan to repaint it at least every other year, depending on the amount of foot traffic.
Before applying a sealer, stain, or paint, be sure to allow the wood to dry out completely. Under normal spring or summer weather conditions, this should take about six weeks. Painting wet wood may result in blistering of the paint as moisture tries to escape.
To slow down decay and keep up appearances, clean the deck annually. Although a high-pressure water spray may be sufficient, it could also remove some stain or paint. Another method is to mix 1 cup of trisodium phosphate (available at hardware stores), 1 cup of powdered detergent, and 2 cups bleach in 2 gallons of water. Use this mixture and a stiff-bristled brush to scrub the deck -- one section at a time -- rinsing with a hose after you finish each area. Use plastic sheeting to cover and protect plants that may be damaged by the runoff.