@ 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!"
This is something I wish to understand better, as well. My only experience with PW'ing is when I worked with a local guy for about 2 weeks last year. He had a 2 150 gallon tanks. One he'd mix bleach in (1:10), and the other was rinse water. Little valve would allow him to switch back and forth between the tanks, and run from the same PW machine.Soft Washing is basically cleaning a surface without using high pressure. A pressure washer can be used so long as the pressure you are using is not going to damage the surface. Thats why I use an adjustable wand anytime I clean anything with my Pressure Washer. There are some surfaces that you cant clean using only an electric or aod pump.