Hey...the best methode for your weathering is, with sandpaper to grind the edges and corners over all the armour and helmet and rub over it with a newspaper (the black color of the letters from the newspaper). Next step is the weathering with black and darkbrown. Dab with a sponge (no, not spongebob ) and the color over the armour and then with a kitchenpaper over the color...especially at the deepst places./ areas. After the weathering you can make a light washing ( heavily diluted with water) with the brown color over all the complete armour.
But with airbrush?...not really
Cheers...