I have a few chisels that developed small spots of rust after sitting unused for a year. I removed the surface rust with fine sandpaper but tiny pits remain. Are these a problem? What is the best way to remove rust? — Monica Raymond
Garrett Hack replies: Pits in the beveled side of your chisels pose no problem — they can be ground out. Pits in the flat back will be a defect in the cutting edge much like a number of nicks. Very small pits you might well lap out before they meet the edge through the course of your usual sharpening and honing the backs of your chisels. If not, then lap the backs with 220 or finer sandpaper on plate glass, and work up through your stones to repolish the back. If you use waterstones, be meticulous to dry your tools, watch for spatters of water that land unnoticed (until you see a rust spot), and wipe your tools with some light oil if you are going to pack them away for any length of time.
Chris Kovacs replies: The pitting will result in a less than sharp edge. Ideally, the back side of a chisel would be perfectly flat with no imperfections. When the bevel is honed on a stone the two surfaces (bevel and back) meet in a perfectly straight line and produce and extremely sharp edge. If there is pitting on the back side, the edge will appear serrated where the back meets the bevel. The serrations are caused by the pits and may only be visible using a microscope or jewelers loop.
The edge may feel sharp and will likely cut well at first, but the serrated nature of the edge will cause it to fail sooner and become dull quickly, it will also not cut as cleanly. Since this problem is on a chisel, I would not be concerned. In most cases, a chisel is not used to produce a finished surface.
If this were a plane iron, I would start with a coarse stone and then move to finer grits until the back side is perfectly flat and polished like a mirror. A plane is designed to produce a smooth surface and a serrated edge produced by pitting on the plane iron will leave you with disappointing results. You will know if the edge of your plane iron is serrated if the shaving come off in strips instead of a smooth, full width shaving.
Jon Siegel replies: If you regrind the chisel you will remove the pits from the bevel surface. But that is not good enough. You must also remove the pits from the back of the chisel. Remember that a sharp edge is the result of two perfectly smooth surfaces coming together. If one of those surfaces (the back of the chisel) is pitted or corroded, then you cannot get a perfect edge. You must lap the back of the chisel until the pits are completely gone.