Painting wheels with an airbrush can really be a PITA if you dont have any commercial painting masks, such as the ones you can get from Czech manufacturer Eduard. In some cases, compas cutters can do the trick, but when you have to deal with small diameters (< 1cm) forget it.
So, I recently developped a technic for that purpose. You will simply need: a cocktail stick, a brand new scalpel blade and some Tamiya masking tape (expensive but worth it).
#1 Paint the rim (was too lazy this day to use my airbrush :P )
#2 Use Tamiya tape to cover the wheel
#3 With a cocktail stick make sure that the tape fits your wheel
#4 Use a sharp blade to cut around the rim
#5 Use the stick to "hide" the slight imperfections and make a perfect circular mask
#6 Paint/remove masks, et voilà ! That simple