Fill the pressure washer's reservoir with an acid-based coil cleaner. Make sure the cleaner is designed for pressure washers so it can go through the pump without causing damage. Wear protective gloves and safety glasses because the cleaner is corrosive. Follow any dilution instructions on the packaging.
Turn on the pressure washer and spray out the cleaning solution on a driveway, parking lot or other location where it won't damage the surface it touches. Stay away from trees and plant life. The cleaner breaks down the scale as it goes through the system. You can also spray the cleaner into a bucket.
Rinse the ground with a regular garden hose once you're finished to dilute the cleaner. If the cleaner went into a bucket, mix it with at least half water before dumping it. Follow any disposal instructions on the packaging.