I use a piece of wood about the thickness of a paint stirrer. Hold it at an downward angle with the wheel spinning. Don't use a lot of force and it will clean off the excess on the wheel. I've never used a rake so I have no idea of how well or how it works.I use a piece of 2x4. If you are getting regular build up then you are using too much compound on the wheels. It only takes a little to get the job done.
The rake I have just leaves a clean buff. I think the black residue comes from metal wearing from the bushing on to the buff, whereas the rake is doing just the opposite...taking off buff material. Anyhow, it works!I've always used sandpaper. The rake must work great since it seems to be the recognized method of cleaning the wheels, but I'm wondering about putting a metal object against the wheel. Wouldn't that turn the wheel black like other metal objects tend to do?