If all you have available to you is your race bearings (installed in the spur, etc) you could always lube them with grease in such a way that they are over lubed. With all that grease packed in the bearing, it is much less likely that you will get the polishing compound into the support bearing. This can later be blown out with some motor spray the same way that the original grease was. Some suitable grease that you may want to try; bicycle chain lube - the kind with high paraffin content, axle grease, diff grease. What you'd be looking to do is form a barrier.
I do agree, though. A second life bearing is probably the best method.