This method of "resurfacing" bearing races by using BB's along with an abrasive grinding/polishing compound comes up now and again.

The problem with this method, which will be obvious if you think about it, is in the convex curvature of the BB's and the concave race surfaces. With ideal geometry the two should mate perfectly where they contact each other, and the contact should be more than at a point. Practically, however, BB's contact their races at over limited areas. That is why BB races tend to develop wear grooves over time. Other bearing types (like cylindrical spindle/needle bearings mated to straight tapered races...like the type used for automobile wheels) have less of this problem (though they have other problems for bicycle wheels, mainly greater rolling friction). Because of this BB/race geometry, it will be very difficult to properly "resurface" badly worn BB bicycle races using the BB/abrasive method. The same applies to BB/crank races.

The best solution is to install new ball bearings and races, if possible. That being said, I have used this method to rescue some mildly corroded/worn hubs for which replacement wasn't possible.