If you are not too concerned with maintaining the patina on the face of the pad, you could try wiping it (just the face) lightly with lacquer thinner. Cleans the crud and surface film off the pad, and it softens the rubber slightly. This also works on the feed rollers of your ink jet printers, in that it removes the glaze formed from feeding thousands of pages and helps the rollers work reliably again until they develop a new glaze.