Hammer Re-Facing, I find is a very satisfying job to do. It has an immediate transforming effect on the hammers and the tone of the piano. Hammer Re-Facing is in effect re-shaping the nose of the hammer. After many decades of playing the hammers become worn and grooved, such that the tone of the piano is lost. Also the grooves in the hammer put a strain on the hammer centres causing them to ware too.
One answer to the problem is to re-shape the hammer head by re-facing. The process involves using a strip of fairly course sand paper drawn over the head of the hammer in systematic strokes. Each time some of the felt is taken away till the grooves are gone and the shape of the hammer is returned to its original 'Olive' shape. This process requires a lot of practice to achieve an even shape. Whatever is taken off one side, the same must be taken off the other, till a nice even shaped hammer is produced.
As said, the result is very pleasing, and produces a much improved tone to the piano.