Many people don't realize the enormous force that's generated at the tip of the wand where the water spray exits a pressure washer. Depending on the power of the machine and the tip you use, the water stream can etch and erode concrete. That should help you understand what it can do to human or animal tissue, composite decking, wood, vinyl or any other material that's softer than concrete. You must respect the machine and what it is capable of doing to you, your pets or your possessions. Spend the first 15 minutes testing what the machine does on things you're willing to throw away.
Protect your eyes
You need to wear safety glasses or even goggles for full eye protection. The blast of water aimed at a driveway or patio edge can shoot up sand, rocks or dirt directly into your face. Paint chips from siding can fly into your face. This happens without warning. If you use a gas-powered machine, wear earmuffs or great earplugs.
Read the guide
Be sure you understand how the machine works. Many pressure washers depend on the incoming water from the garden hose to cool the pump that creates all the pressure. If you have the pressure washer on, but no water is flowing through the wand, you can damage the pump. Read the owner's manual or any quick-start guide.
New pressure washers typically come with an assortment of tips for the wand. These can range from 0 degrees up to 40 degrees. The lower the number, the more concentrated and dangerous the spray of water. Start cleaning the house siding with a 40-degree tip to see if it achieves what you want. You'll probably have to use a 25-degree tip to get good results.
Watch your aim
Never aim the spray wand up as you clean siding or aim it at cracks where siding butts up against windows, doors or any trim boards. You can drive water deep into these cracks and cause leaks or damage to your home. Never aim it at vinyl siding overlap seams because the water stream may get behind the siding.
Remember, siding is installed based on the fact that Mother Nature's rain aims down, not up. In rare instances, rain may be horizontal, but seldom does rain blow hard in an upward fashion. It can happen because of the shape of roofs and dormers during violent windstorms or hurricanes, but it's not normal.
Realize that pressure washers can readily peel paint from just about any exterior surface. If you want to use it for this purpose, hold the spray wand so the stream of water attacks the paint at a very low angle. The spray wand would be nearly parallel with the painted surface.