A few options come to mind. First is using weathering powders followed by a coat of matte finish to seal them in. I've actually used powdered tempra paint with a good degree of success to get that slightly faded look you're thinking of. I mix a bit of white into black so when it is sealed, it's just a darkish grey.
Another option would be do use a wash of acrylic paints as I describe in the current (June '08) issue of Garden Railways. This lets the dull greyish-black paint settle as needed, but still lets the slight gloss of the boiler (or whatever surface) show through--something that you can't get with a matte overspray.
Later,
K

Another option would be do use a wash of acrylic paints as I describe in the current (June '08) issue of Garden Railways. This lets the dull greyish-black paint settle as needed, but still lets the slight gloss of the boiler (or whatever surface) show through--something that you can't get with a matte overspray.

Later,
K