The best tool I’ve found for sharpening in the field is the Sportsman’s Steel, made by Gerber. It is now discontinued but still easy to find online. An elongated steel wedge 5 inches long by 7⁄8 inch wide, it pivots in and out of a leather case. The steel is coated with superhard chromium carbide. Learn how to stroke your blade along its side, and your knife will be sharp enough to shave with even after you’ve just butchered a bull.