@ Joseph - What most people use are diaphragm pumps.. delavan, shurflo, udor to name a few. They are either air-operated or 12-volt electric driven. Try out Roof Cleaning Institue of America/
http://roof-cleaning-institute.activeboard.com There are lots of great info on pumps on that forum.
@ Javier - From what I’ve learned over the years, "soft wash or soft washing" is a cleaning method using pressure of no more than 300psi, give or take. For example, in Roof Cleaning I use a 5.0gpm delavan pump to apply my 'roof cleaning mix' by spraying on a chemical solution to help/ad in killing off the living organisms, aka
roof algae. Another way we use "soft wash/washing" for power washing is the same method as roof cleaning, but reducing the chemical mix formula, by help lossing the grime and dirty, followed with power washing the surface without high pressure. Charging your tip size helps reduce the pressure also!
AC Locker or Chris Tucker has been in the industry much longer then I have, and they would know the true meaning of “soft wash/washing!"