How hot is the water comming out of the tip, about 200 ? I can see how the heat softens up the tar/undercoat and the pressure blasts it off.
I am going to rent one to do my car.
I cleaned my engine bay with a just a pressure washer (no steam) and it was a black painted spray bomb job that some Chevy guy did before I owned the car. The pressure washer with a narrow tip removed 90% of the black spray bomb paint off without taking off the factory paint that was underneath.