Pressure washers are not ideal for cleaning composite materials, but they can be safe if used cautiously, and the pressure is no greater than 3100 psi. Pressure washers are great for removing dirt and dust but not for stains.  To use the pressure washer safely, spray the fence with soap, gently scrub it, and then spray each board with a fan tip at about a foot’s distance from the surface. Rinse thoroughly next, as dirty water left on the surface will dry as a grimy layer.