Build up is welding in new metal, then grinding it back to the original size. Its a poor way to repair the damage, but without pulling the crank apart its the only way. It does have some advantages, the pin will be welded to the webs, and provided they manage not to warp it it should be fine. White metal shells a very soft, its the finish that is important rather than surface hardening, but that can be achieved with carbon & heat treatment.
Have fun
