The answer to 'why is my pressure washer stained' is that pressure washers can become stained over time due to a buildup of dirt, grime, and mineral deposits from the water used. This can cause discoloration and a worn appearance on the exterior of the pressure washer.
Some common causes of pressure washer staining include using hard water, not properly cleaning the machine after each use, and leaving the pressure washer outside in the elements. Staining is often more noticeable on lighter-colored pressure washers.