Well Chris, it is not easy to explain this technique, but i will give it a try..
I discovered a strange method, but it worked for me....
First i sprayed the entire frontfork `matt black`.
Second, i made a water/pastel mixture of `orange rust` from the Tamiya weathering kit `C`.
Then i brushed on the mixture on the frontfork, and let it dry.
Theni made a mixture of brown oil paint with white spirit.
Start applying this mixture with a soft, fine brush, be careful not to damage the pastels with the brush.
The strange thing is that because of the structure of the pastels, the oil paint seems to dry and get a matt finish after seconds.
Because of the rough structure of the pastels, the result is a rough surface, same like real rust.
There is one caution; after doing this, be very carefull not to touch it with your fingers because it is very vulnerable for damage from your fingers.
That is why you have to seal it of with a thin coat off matt transparant, to protect it against your fingers and further weathering.
After this proces i added the paint with a soft brush. Be careful with applying the paint because the surface is still vulnerable.
When the paint is dry, small chips of paint can be scraped away with a pointy scalpel knife, and the rusty pigment wil show up beneath the paint.
The keyword for a good result is practice..
I also used this pigment/oilpaint mix technique on the exhaust of a 1/9 scale German Zundapp.
Karel