it`s very difficult to explain the weathering because i use many different techniques to achieve this finish.
In case of the battery (pictures below); first of all i paint it the base color, Olive Drab (enamel)
Then i apply the light colored orange-rust pastel from the Tamiya weathering kit.
After dry, i apply thinned Raw-Umber oil paint (70-30%) directly on the pastels, which give the dark rust colors.
Strangly, because of the pastels the oil paint dries immediately (at least it looks like it because it gets matt/dull the moment the oil paint mixes with the pastels)
By adding more or less thinnner (white spirit) you can change the rust shades.
Then i spray on a coat of matt clearcoat to seal the paint, because the pastel/oilpaint mixture is very vulnerable.
The little rust dots i apply with a tiny piece of sponge.
Forgive me for writing/language faults because i`m not writing English that often
The battery is ready and mounted