I used to use an old junk turntable with the center post removed, and replaced with a straight pin on exact center. Then I would use some bushings to fill the hole in the wheel and keep it centered on the pin. Then I nounted a small brush on to the end of the stylus, and repositioned (Bent) the tonearm so that it would reach the center. THen I just put a small amount of paint onto the brush, and lowered the tone arm with the cue lever, just like I was playing a record. A couple of hits around to make my edge, and I could paint the rest freehand, or with a less precise mask. I ended up pitching the turntable out last time I moved, so I had to find an easier way to do it with less
Now, I just freehand it, or make a conical mask out a label. sometimes, I just make a mask out of plastic striping tape that sits 90 degrees to the face of the wheel. THis works really well when there is a well defined rim. WHich side I paint first, and which way the sticky side of the tape faces depends upon what type of features I have on the wheel, and where I have to make the line between the rim and tire.