I recently found some old sharpening stones in a garage sale. They were covered in oil and looked horrible and everyone passed them by. They had glossy gray streaks showing metal buildup. I paid $3 for six stones.
Since the sharpening stones appeared to have been used with oil, when I arrived home, I took a spray can of brake cleaner and sprayed them well. After letting them soak for a few minutes, I washed them off with a garden hose. They came out almost as good as new, all the oil and buildup metal in the stones was gone. Most of the metal filings came out of the pores with the oil. I have been using these stone and there are working well.
If you don’t have brake cleaner, an alternate method is to use honing oil. Honing oil normally provides lubrication, reduces friction and keeps metallic particles from embedding into the sharpening stones while you are using it to sharpen a knife. To clean your sharpening stones, apply a small amount of honing oil, and rub it in a circular motion with a small brush down the length of the stone. I normally use a fingernail brush. You’ll see metal flecks rise up from the pores, and as this happens, wipe the stone with a rag or paper towel. Rinse the stone under running water and then dry it with a paper towel or rag.
If you have a water stone, you can clean the sharpening stone with warm, soapy water. You can use an old toothbrush, fingernail brush or a green scrubbing sponge to clean the stone. Use a circular scrubbing motion. When you see the metal fragments, start to rise, wipe them off with a damp towel.
Occasionally you may encounter sharpening stones you have to clean more than once, but most will clean up surprisingly easy. Just be sure to put a towel under your sharpening stones during the cleaning process to catch excess particles. Especially if you do it in the house.
If you take the time to clean a sharpening stone and protect it from damage, a good stone will last a lifetime for most people.