I have another idea on oxidized metal simulation that you might want to give some consideration to. Oddly enough I learned this through trial and error trying to simulate gravel in dried mud stuck to a 1/72 scale tank.
First, mix your paint until you acheive the "rust" tone you want and then paint a smooth base coat of that color on the components you want to be rusty in appearance.
Next, crush some pearlite (available in most well stocked gardening supply shops) until you get it to a consistency of something like baking soda. Mix the crushed pearlite with some of the rust colored paint in a saucer, or other largely flat surfaced container, until all the pearlite is covered completely with the paint. Once this is done, thin the mix to the consistency of milk just as you would thin normal paint for airbrush use.
Now, take an old soft bristle toothbrush and dip the tips of the bristles into the paint making certain that some pearlite is picked up in them. Point the bristles downward and drag your finger along the bristles so that when your finger moves across them they'll spring back and spray the paint forward onto whatever it is you want your paint on.
You'll want to spray the excess paint off the brush onto a paper towel or something before aiming it at your model.
I found this method gave a very convincing "gritty" effect to the paint when I tried to simulate gravel in mud. As rust is somewhat gritty itself, this may be a method you'll want to experiment with. The smooth under coat I mentioned earlier will provde a consistency to your finished paint job that using the pearlite and paint mix alone will not capture so well.
I should note that I use acrylic paint almost exclusively in my modeling, so I cannot attest to how well this method might work with enamels.
Hope it works for you