Pigments have been used to simulate the rusty surface of the exhaust silencer assembly.
The technique is to pick up pigment particles from the lid of the pot rather than plunge the brush into the pot itself. This means that the finest particles are being used to apply to the subject. The brush used is a filbert, chosen for its soft but firm bristles and rounded shape. I like soft, firm and rounded. Only small quantities are used, because it is only a suggestion of rust, and not a flaking, encrusted pile.