Yep, pressure washer good,but be aware, you will be washin ****e outa yer chassis for weeks if it's not bin done before,as you always miss bits,lol.Also car ramps are good fer getting right underneath.
I had the chassis steam cleaned before I recoated it with GM undersealant. The steam clean was really effective and the chassis was spotless afterward. It also meant that the guy at the garage where they did the steam cleaning was able to spot a couple of little rust spots that needed welding near the back.