If you spray with water/foam cannon only and then don't rub it of any kind (washing or drying) it will get most of it off, but it will leave a film behind. The film may be depending on how salty your car is, but I tried it a time or two this winter when I had salt caked up to my windows and it ended up leaving a layer of film. The film looks better than having it white to the windows though!
On a side note, this film helps you figure out where you are slacking when washing the car :laughing: . Since I blow dry the car, when it is dry I can see little areas of film left where I got lazy, usually down on my lower panels.