The main work has been done, and the airbrush can be set aside for a few steps.
Weathering powders, or pigments as I tend to call them, come in a great variety of textures and colours. The ones that work best for me are from military modelling ranges and are exceedingly finely ground. This enables the particles to fall into the lower reaches of the matt surface's layer(s) and stay there beyond the reach of fingers and other abrasive effects. In this case I'm using MIG Productions colours Dark Mud (for new rust), Track Brown (for old rust) and Black Smoke (for accumulated oil and grease).
I apply the pigment using a filbert brush, and collect the finest particles I can from the lid of the jar. The excess is removed by tapping the brush on the edge of the jar and the pigment is applied by gently touching the bristles to the target surface. The result will be a barely perceptible discolouration of the surface, and more intensity can be achieved by repeating the process.
The reasons for the choice of brush are that it is designed for use with acrylics, so will be pretty tough, the bristles are fairly soft but quite firm and it is a pretty shade of yellow. You will note from the photograph that the bristles have lost their shape, but it has been in use for some 5 or so years.