No deck preservative will work effectively unless you apply it when your deck is clean, and prepping a deck for its periodic coat of waterproof sealant can be a big job without the right tools. Many of the pros use pressure washers together with cleaning and pretreating products designed to lift grime and remove loose wood particulates. One of the most popular and effective pretreaters these days is oxygen bleach. It has a foaming action that helps cut down on the amount of scrubbing required and is an environmentally friendly and landscape neutral choice. Chlorine bleach and TSP (Trisodium Phosphate) are other popular options, but both can be caustic and have a negative impact on the environment.
There's some debate among the experts about the use of pressure washing equipment to clean wood. Detractors say that high water pressure destroys the surface of many common wood products, leaving wood so rough that it may require resanding. Proponents claim that the wear and tear is minimal while the advantages of a quick cleanup make it worth the risk. If you do go the pressure washer route, use the lowest setting that will do the job. Try starting with a pressure of around 500 psi (a rating of pounds per square inch), and work your way up from there. Read up on using a pressure washer on wood, too. Cleaning spindles and stairs may require a shorter wand for tight corners. Maintaining a smooth, even motion is important, too. When it comes to deck cleaning, good wand work isn't just reserved for Harry Potter and the wizarding world [source: Haege].
After your deck is clean, give it plenty of time to dry completely. Three dry days at temperatures above 40 degrees Fahrenheit (4 degrees Celsius) will insure that the deck is dry enough to apply sealer effectively [source: How to Clean Things].