For ease of priming, the wheels are temporarily put back onto their axles which are masked off. I don't want primer or paint on the axles at this stage as they've yet to be fitted to the inside bearings.

Any flash on the plastic spokes has been pared away with a Swan Morton blade, files and wet & dry paper - whatever is required. Of course the camera picks up no end of evils which the Mk.1 eyeball overlooks...

If, as in this case, the final colour of the wheels is black, then they are primed with a black etch primer from an aerosol. My current lot is from ebay and the coverage is excellent and with this primer I no longer need to bother with a top coat so with one less coat of paint the detail remains crisp. Wheels that will be painted other colours will usually be primed in white or light grey before having a top coat applied.

Wheel front and backs are given a short burst from the well-shaken can, and the solvent in the etch primer flashes off quickly leaving a very smooth slightly egg-shell finish with a nice dense colouration. Within a few minutes the primer is dry enough to handle so the masking tape is peeled off (I ought to have shares in Tamiya masking tape) and a cotton bud moistened with cellulose thinners run over the tyre treads. A dry bud quickly follows to remove all traces of primer. The treads could be given a once over with the brass burnishing mop at this stage, but I usually leave it until the weathering has been completed.

As I noted in the comments section of the last post, the chemical blackening of the tyre treads inhibit the natural rusting process of the steel to a negligible and controllable manner, but another benefit of the chemical blackening process is that it it etches into the surface of the metal and means that, for example, the axles will not need priming before weathering. The blackening process keys the metal ready to accept paint, and the only reason I prime the wheel faces and backs is because these areas tend to get some rough treatment. It's no more than a belt & braces approach. The first time I heard about this property of the blackening process was Raymond Walley's website in his build of the Flying Scotsman for the NRM - see the part about painting the chassis where he chemically blackens the frames before top-coating, omitting the primer.