A variation on the playing card theme: cut some strips of stiff tag board (like sand paper sheets are packed in), tape them together, circle them around the edge of the rim, clamp with a paper clip and spray away. I've been using the same piece of card board for about 20 years now.
While I'm at it, while I wouldn't slather paint on the lug nut/wheel contact surface, a little paint won't make the wheels fall off. After all the wheels come from the manufacturer painted or powder coated. Besides, by the time you torque the lug nuts to their 400 to 500 foot pound requirement a light coat of paint will be all but squeezed out. That along with your normal regular attention to lug tightness and you'll be good to go.
I dragged out my old masking shield and slapped it on an old T-bird hubcap just to show it off.