Stop off at your local hardware store and pick up a small roll of aluminum roofing flashing.
It will fit nicely in the area betwen the rim and tire and keep the paint off the rims. I just cut a strip of it long enough to go around the circumference of the rim. Paint away, and then use it over for all four (or as many as you have) rims. You will have to use a small brush to do the edge right against the tire, or tape it.
I've also cut a wedge of it and curved the edge to fit the rim, then worked my way around the rim as I painted.
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