I wanted to interject that some of the washing rags people use are shop rags or wash mits that had another type of soap or fabric softner in them. Alot of car soaps have silicone in them and if you use the same rag or brush that will cause a problem. For using towles or rags that have been through the laundry there could be residual fabric softner in the material.
My little shortcut is take one of those pump spray jugs (approx 2 gallon size) and fill it up with hot water and add a fiew spritzs of dish soap. Pump it up and hose away then rinse.