I used to operate a pair of exterior express car washes. Our “prep guns”, using an alkaline soap, used Hydracell D-10 sealless pumps (like this one: http://www.hydra-cell.com/product/D10-hydracell-pump.html) and while they were capable of much higher pressures, we had them set to about 900psi. The trick is in the nozzle aperture size used, and how close you train your employees to keep the end of the lance away from the car’s front end. In the summer (bug) months, you NEED that pressure (and the chemical) to loosen/remove bug remains from the front end and windshields of the cars before they enter the tunnel as the wash itself will not generally do a good job removing them.
The other part of the training is getting teenagers to pay attention to existing paint blisters/rock chips on painted bumper covers because if you get too close, it WILL enlarge them or start removing paint. The default position if that happens however is that it was an exacerbation of existing damage (which it is) and that we weren’t responsible for pre-existing damage.