We bought one of these "foam guns" this summer & they work great, I think. You fill this little canister with your favorite soap concentrate, depending on what you're washing, and then clip this on the end of your power washer wand, first removing whatever tip you have on the wand. This particular one is made by Mi-T-M out of Iowa who makes power washers, air compressors, etc. The gun will lay on a thick layer of soap foam on whatever you want. It'll be 2"+ thick if you want. Then, shut the washer off & let the soap work for 10 minutes. Come back w/your washer & blast off the residue. Works great here. The best thing is, the soap WILL come out.

One minor problem with operating the gun. You do have to "clean" the injector after every use so that soap doesn't dry in the venture. Instructions warn of that. I've found cleaning to be very easy. Just unscrew the soap container & stick the siphon tube in a jug of water & turn on your washer wand for 1 minute or so & let it suck clear/clean water thru the gun. Works fine for us.

Looks like the gun is $60+ at most places. I'd buy another one tomorrow if it were needed. Works great on machinery, cars, restored tractors, etc. Just choose your soap according to the subject at hand.

Here's the gun on Amazon which is where we bought ours:

http://www.amazon.com/Professional-Foam-Lance-Adjustable-Bottle/dp/B004GNZYY4/ref=sr_1_3?ie=UTF8&qid=1411467050&sr=8-3&keywords=foam+wash+gun