My set of Norton waterstones has recently developed two problems. First, a brownish stain appears when I sharpen my chisels and plane irons. Could this be rust? My second problem is mold, probably the result of storing my stones in damp conditions. Can you recommend a way to get rid of the rust and mold?
You’re right about the first problem. The brown stain is from the oxidation of small steel particles. This doesn’t affect the functioning of the stone and is only an aesthetic concern.
To prevent mold from growing, put a few drops of chlorine bleach in the water that you soak your stones in. To clean up moldy stones, you can lap them on a piece of 1/4-in. plate glass with either 180-grit silicon carbide sandpaper, or 90-grit carbide particles (available from auto parts stores). Despite any remaining stains, the stones should function as well as ever. From time to time, your waterstones will need further lapping. This keeps the stone flat and clears it of any clogging particles.