One of the things you can do is to give the model a cover of DullCote or another flat finish. Then, mix up some highly thinned oil paint and brush it on.
What color of oil paint, Tom? I would have a tendency to use a white or lighter version of what you are fading.
I have done something similar using DullCote to give the surface with a bit of a tooth then dust on some white chalk (or weathering) powder with a stiff brush then hit it with DullCote. Repeat until you get the fade you desire. This technique also allows you to control where you want the fade, much like you could do with an airbrush.