One of the small garages I worked in had a quite comprehensive car wash set up complete with a spectacular chassis wash which consisted of high pressure jets buried in the run up to the wash itself. If you chose the chassis wash option, in addition to one of the wash options, these sprays (capable of projecting water about 2 stories high if no car was over them) would activate shortly after you pressed the "go" button and you slowly drove over them on your approach to the wash. There was always bits of underseal lying about this area!
The car wash area was often a very entertaining area to watch with drowned engines, (this was when cars had points type ignitions) Radio aerials, windscreen wiper blades and even door handles being ripped off (door handles, conveniently for the machine, stuck out in those days - remember the old mini handles?). Even if no obvious damage was inflicted it was interesting to look closely at the paintwork of any vehicle which used the wash frequently - convinced me to never go near one.
I am also a believer in hand washing. I bought a two bucket washing system just after I bought my new Ibiza. 2 large buckets with dirt barriers in the bottom. A couple of noodle type washing mitts. Big and nice wee alloy wheel brushes and a couple of seamless microfibre drying cloths. I always wet the car first to loosen dirt and then, maybe ten minutes later, hose down with mains pressure water to loosen grit and bigger bits of dirt. Then the two bucket system. One (labeled Wash) contains diluted car shampoo the other (labeled Rinse) clean water. The noodle mitt goes into the wash bucket and then you wash a small area of the car. Next into the rinse bucket for a wee "shuggle" to let the grit drop to the bottom and clean the noodles then back into the wash bucket to pick up clean shampoo water. Work from top to bottom and front to back. So you are working always cleaner areas first then towards the dirtiest (like sills and tailgate). Then a general rinse with the hose nozzle set to a medium spray. Sounds complicated but once you get into it it's actually quite quick and 3 years down the road "Twinkle" (the Ibiza) is still sparkling! It's amazing how much gritty residue ends up in the bottom of the rinse button. Without this system these little bits of grit would be staying in the washing mitt and inflicting minute scratches in the paintwork.
I watch, with horror, my neighbour who turns his pressure washer on the car about once a month, if it's lucky, as he blasts high pressure water into every orifice - Radiator grill (goodness knows what damage that's doing to fins etc) heater air intake around the wipers (bet that water finds it's way over ECU's etc) Into door panel gaps (probably washing the lub off the hinges and locks) Oh dear I could go on couldn't I?
