The answer to tough stains on pressure washing is that there are a few common causes, including built-up dirt, grease, or mold. Symptoms may include stubborn stains that don't come out even with high-pressure water. The best solution often depends on the type of stain, but typically involves using a cleaning solution or detergent designed for pressure washing.
For DIY pressure washing, be sure to test any cleaning products in an inconspicuous area first. If the stains are very stubborn or you're not comfortable using chemicals, it's best to call in a professional pressure washing service. They have the right equipment and experience to tackle tough stains safely and effectively.