You can attempt to sand it all down even, but even then that very well may still show thru forever. The power washer etched or removed the wood in that pattern.

Things such as that are much harder to fix than to mess up so who are you planning to do the fixing?

More abrasion is the only solution. Fortunately, they don't look too deep. $500.00 Festool 150 on grind mode and you're done in 10 minutes. $49.00 palm sander, 10 hours.

You might be able to rent a random orbital sander.

Please show this to your handyman, so he knows to be more careful with a pressure washer in the future.