I was one of the first people to go on a training course for KEW washers (a fluke - I have never worked on them since as the firm I worked for went under a that time!) and they said you should never use a pressure washer on a car. OK, they later brought out washers suitable for cars - but only if the pressure is cut down, never full force!
I bought my first pressure washer in 1996 or so and used it to pressure wash under Ma's '89 Metro twice in its first two years. I soon stopped as it was rustier than the previous one that had never seen the washer in five years (and did more miles on our salt-water affected roads). I wouldn't dare wash under a car with my current washer at full pressure although I occasionally wash the Golf with it: mainly using the brush but never anywhere near full pressure! I have used full pressure on the tractors - and removed the paint (including some B&Q gloss painted over B&Q metal primer!).
A good hose pipe, with plenty of flow, is far better for washing under a car.
The black Waxoyl I mentioned is black Waxoyl, not underseal - there is a difference and I hadn't spotted that Grumpy2 had mentioned underseal
I bought black because they didn't have any clear stuff and I wanted it on some hydraulic rams to stop them rusting in the winter weather. As you have to clean it off before using the rams, it didn't matter what colour I bought. Although I have some underseal somewhere, I can't remember when I last used any - it was for a shed or something non-vehicle related! Come to think of it, I haven't used Waxoyl on a car since about 2005, a couple of years before Ma sold the Metro!