Always check with the manufacturer of the composite to make sure if you can use a power washer. Some manufacturers didn’t recommend their use. Others have very specific guidelines.
Generally power washers should never be used over 1500 PSI! A fan tipped nozzle should generally be used, never closer than approximately 2′ from the composite. Composites can easily be permanently scared or damaged.
Power washers should never be used with Corte*Clean®. Power washers are only good for removing loose dirt, debris, leaves, etc. generally several days to a week or more before using Corte*Clean®, to allow the composites to thoroughly dry. Do not try to blast fungus stains out of composites! Not only will you likely permanently scar the composite, you will drive the stains deeper into the composite, generally making it cost more time and money to Corte*Clean®.