The next step is to randomly remove some of the paint to portray a buildup of grime around things like coupling hook, bolt heads and pipe connectors. This is achieved by using a slightly damp brush, and by that I mean a couple of drops of thinner placed on the bristles rather than dipping the brush into the thinners. The brush is then used in a stabbing motion, rather than a stroking motion, to slowly and gently remove the paint. If it all comes off straight away, your brush is too wet!
Another method is to use a cotton swab that is damp and perform the same actions, although this will inevitably leave some cotton strands about, and you'll forget to blow them off before taking a photograph . . . . . . .