No real secrets I'm afraid! I don't use pigments or the like!
Chips were done by brush using a dark gray with a bit of silver if big enough. Rust was done using cheap acrylic raw umber thinned to a wash.
Oil was just thinned black.
Devlan Mud is the secret to the depth, apply liberally bUt then clean from large open panel with a cotton bud to prevent pooling.