I can't take credit for this excellent fading technique, but I will explain in more detail about how I personally mix the fade "cocktail" and what materials I use.
For a simple white fade, I use about a marble sized amount of Liquitex brand acrylic paint. Be sure to buy the "transparent" type of paint, and not the opaque. I bought mine at a local Hobby Lobby store. I am not an artist and don't know a lot about paint, other than the obvious water based vs oil based facts etc. What I have observed is that this particular brand of paint has a transparent, translucent, and opaque, for just about every color hanging on the display. If you choose to buy other colors to tint the mixture like I have done, the transparent labeling is very important. The transparent property of the paint allows it to "fade" and "tint" the car, rather than "paint" or "cover" the car as an opaque paint might do. Ok, I sound like I know what I'm talking about don't I?
I use a medium sized paint jar to mix the fade coat, and add roughly equal parts of Micro Flat and washer fluid to the jar. I leave lots of air space in the jar so I can vigorously shake it to dissolve the thick acrylic paint. It will look like 2% milk when ready to spray with your airbrush. I mixed a batch too thin and it took many many coats to do a heavy fade. It's best to experiment and get the right mixture for what you plan to do. The reason the Micro Flat is added is to protect the coat that was just applied. In the past I applied a diluted white paint with varying results, and would follow it up with Testors dullcoat. Having the Micro Flat in the mix saves a step and avoids the fade from getting rubbed off or scratched during handling of the car. I have played with the amount of Micro Flat in the mixture as well, and again depending on how you perform your own weathering, you may want more or less. I find that I generally add a top coat of Testers dullcoat on the finished car after all weathering steps, or is a must if I plan to use oils or mineral spirits during the oil steps, to protect the layers beneath. So in short, if the fade is the only weathering step, you may want more Micro Flat. If the plan is to add additional weathering layers, you may not need as much Micro Flat in the mixture.
Here are 2 identical boxcars that I was working on tonight. I applied graffiti to one of them, and safety striping to both. The front car was given about 5 very light coats of the fade mixture. I use a dual action airbrush which is great for this application. Since the mixture is very thin and "watery", I like to make a spray pass over the car, and then blow some air from the brush to evaporate the liquid quicker. This allows me to immediately go over the car with subsequent light coats all in one step.
There you have it. This fading technique is very fool proof and difficult to mess up. Really the only way I have found to mess this up is to spray too much at one time so that it begins to run. In the past I have tried to thin Polly S reefer white acrylic, but found that it went on to grainy or splotchy. With this fade coat, it really goes on super smooth. Reminds me of that Keystone commercial: "Keith Stone, always smooth".
If you try to technique, please let me know how it works for you and share some results.