I had the same problem. The Norton stones are very porous and grit from the abrasive paper is now embedded in the stone. I have lapped several other stones with abrasive paper (coticules, Chinese 12K, Belgian Blue and barber hones) without problems but the Nortons don't tolerate it. The solution to my problem was to lap it on a Norton flattening stone. Other people use diamond hones, such as the DMT 8C. Since I started lapping on the Norton flattening stone I haven't had any problems.