Thanks gentlemen for your kind comments! As I'd mentioned - this weathered look is still a work in progress and will probably look a bit more toned down when the model is finished.
To answer "Umlaufmotor" about what I used to create the effect so far, is a base "wood" coat of acrylic, then glosscoat, then a light coat of hairspray, then a mottled, patchy coat of the XF-71 dark green. I scrubbed off some of the paint with a damp brush, since the hairspray dissolves and the XF-71 looses it's grip. It was an armour modelling technique I wanted to try on a WWI aircraft, and since this one had such a monotone colour for the fuselage I really needed to make it interesting. The trick will be to make sure all components have the same look of wear and discolouration when it's all assembled.
This same technique (using a base of Alclad II aluminum) was used to do the cowlings - here's a pic of one without the scrubbed-off paint and one after treatment...
Cheers,
Gary