Lots of good suggestions here. What I'll try next to an overall spray of a reddish-brown rustoleum like paint and then dry brush with steel acrylic. I need to dig out my cheapo air-canister powered airbrush as see what I can do with it. I've never yet used an airbrush on a model. And I agree with Dick that WWI models generally look better if hand-painted. As far as the NZ replica being "too clean," well, let's say it's at the beginning of its service life!
(Incidentally, weren't most aircraft actually stored in tent hangers? Can you really just leave a plane like this out in the rain?)
Foodstick, I do have some pastel chalks that I can grind up and play with the powders.
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I coiuld also try TFF's idea of an overall "steel" paint with a very light dusting of the reddish brown.