If the edge of the blade is badly chipped, you may want to "thin the edge" before you sharpen it. This means removing metal from the side of the blade. This is most easily done on a belt sander by carefully laying one side of the blade flat on the moving belt and pressing gently for just a second or two, then cool the blade in water. If you don't let the blade get too hot to touch you won't ruin the temper. If there's a motor pool nearby there might be a belt sander in their repair area. The alternate method is to use coarse sandpaper on a flat surface. Any fairly flat surface will do, even a piece of wood, but something really flat like a cast iron machine bed, or glass, or a polished stone tile, is better. Some folks use a cement block, abrasive and flat surface all in one. I've done this once, it took about 3 hours and 30 sheets of sandpaper to do a 3.5" blade. It only takes about 10 minutes on a belt sander with a fresh belt... Once you get through with the coarse abrasive, use successively finer grits to remove the coarse scratches on the side of the blade, and then to sharpen the knife