Like anything exposed to the elements, your outside wood surfaces get dirty and need a good cleaning from time to time. These can include decks, fences, and even outside walls. The best time to perform wood cleaning is before damage starts to appear. Loose nails, warping, and fading are all signs of trouble. You will want to pressure wash your wood to clean it, and then treat the wood to prevent damage.
Use your pressure washer to clean your wood, using a cleaning agent. It will not fade your wood the way bleach does, and its use makes it very easy to perform any wood cleaning that you need to do. Not only will it clean the wood, but it will brighten it as well.
You will probably want to use a cold water pressure washer - never use hot water to clean your wood. The pressure washer should have an output of at least 103 to124 bar (1500 to 1800 psi). The tip that you use is very important, too. A 0° tip will cause damage to the wood, so don't use that. Instead, use a 25° tip, which will provide a safe yet thorough cleaning.
Never hold the tip directly at the wood in one place while wood cleaning. This will damage the wood. It's best to practice at first on old pieces of wood that you can discard. Once you get the hang of it, move to the real thing. Clean three to four foot sections of wood at a time. If it is hot and sunny out, be sure to rinse off any cleaning solution from the wood every 10 to 15 minutes, and then go back to your cleaning. Repeat this until you are finished, then go over the entire surface again with fresh, clean water.
After your wood cleaning is finished, always allow 24 hours for the wood to dry before applying any protective finish.