Pressure Washing uses a pump which forces water from a nozzle at pressures up to 400 bar.
This method is good only for loose rust, paint and contaminants. It is very good for cleaning non oily areas. For oily and greasy surfaces an emulcifying chemical must be used in combination and preferably with hot water.
It can be an acceptable surface preparation in ballast tanks but only if used in combination with power tooling, grit injection or if a “soft” coating is to be applied. Soft coatings are frowned upon these days and not accepted by Class in some ship catagories.