I like using pencils to replicate the bare metal where paint has been worn off a sharp edge. I use burnt cienna pastel chalk rubbed around the edge to simulate older rust, then a lighter shade even closer to the edge to simulate new rust, and with discretion, I lightly rub the edge with a regular No 2 pencil. This does a good job of showing an area that recieves a lot of wear. Martin's technique works well to do larger areas of worn metal, like tread plate and floorboards. Pencils are good for showing scratches too, just use a really sharp point to put a scratch anywhere a hedge or branch might have dragged along the vehicle.
Rob