When I painted my GS wheels years ago, I cut out a stencil that covered just the outside of the rim, masked the center off. Then I gave it a thin coat of paint, then wiped off the chrome (where it wasn't supposed to be painted) with a rag with a little bit of thinner on it (just a small amount so it wouldn't run). Repeated this a couple of times on each wheel, turned out great. Much faster than masking the entire wheel.