I use Silly Putty to mask the rims while painting aircraft tires.

First, I spray the entirety of the wheel with white primer. Next, I roll small balls of putty and push them over the wheels until the edge reaches the rim. Then, I then spray the rubber color over the tires.
 
Finally, I remove the putty, leaving a nice round rim in the middle of the black tire. It couldn’t be easier to paint white wheels.

Tip submitted by:
John Hofherr
West Fargo, N.D.