A belt sander, with hundreds of revolutions per minute and dozens of available grits of paper, can put an edge on a knife that only hours of manual sharpening can achieve. Professional knife restorers typically use belt sanders, moving from coarse to very fine grit, then to polishing belts made of leather, for the finest possible edge. You can achieve the same results at home with practice and a stationary belt sander. (A hand-held belt sander will not give you the consistent edge that you require.)
Load a 150-grit or 240-grit belt onto your sander. Use 150 if your knife is extremely dull (does not cut at all), and 240 grit if it has a decent but dull cutting edge.
Position the knife blade with the blade pointing in the direction of the belt. This is unlike using a whetstone, in which you lead with the blade on the sharpening surface--more like stropping a razor.
Maintain an approximately 20-degree angle between the blade and the belt.
Draw the blade across the belt, on one side. Work quickly, grinding one edge no longer than five seconds. You are removing metal, rapidly. Keep the edge of the blade perpendicular to the belt; thus you will have to use a curving motion, so that the edge of the blade points toward the floor.
Feel the blade once you are done; if it is too hot to touch, then, you have oversharpened the edge.
Repeat this process on the reverse side of the blade.
Repeat sharpening, moving down to finer grits. The progression should be 150 to 240 to 400 to 20 micron to 9 microns to leather and honing compound.
The progression from 240 grit to 9 microns is sufficient for most uses; the leather and honing compound is necessary only for straight razors, scalpels and restoring antique blades for presentation.
Test your sharpened blade. It should slice easily through a piece of paper, starting at the edge, and holding the paper with only one hand. If you have used the leather and polishing compound, you should be able to shave some hair off your arm.
The blade should appear brand new at its cutting edge.