I use the same technique for 15 years! first I roughly mix black and leather paint with a bit of water -quite a bit in fact, the mix is pretty liquid, then i apply the dark mix precisely where the rust will be. then, when the paint is still wet, i blend some leather coloured paint unevenly on the top of the mix. then -with a paint still wet -I load some pigments on the brush that i dried more or less with a tissue, and i lightly touch the surface. The trick is done!