This is not exactly a "fade", but it may give you some ideas of what you can do by angling the airbrush.
There was no masking on this car. I did it from the bottom up, gray, light blue, dark blue. By angling the airbrush so any overspray would go off into space, I was able to let the body lines separate the colors. I think if you practice a little which way you aim your airbrush, you could do a fade without masking, as Ace described.