To truly sharpen your flat-bladed tools, I recommend investing in some Japanese water stones. These come in a variety of sizes and hardnesses. True to their name, they should be soaked in water for at least ten minutes prior to working with them. In the photo above, you may see (from left to right) an adjustable base support, a lap stone, a coarse Japanese water stone (240 grit), a combination stone from Joshua Roth (280/1500), and a combination stone (800/4000). All these items and more are available from Woodcraft. I use four stones in succession, 240, 800, 1500, and 4000. Woodcraft also sells an 8000 grit polishing stone.
In order for your tools to be sharpened effectively, your sharpening stones must be perfectly flat. The only way to do this is with the lap stone, which is far harder than your hardest stone and yet coarse enough to remove the high spots. This stone is soaked, too, and the smaller sharpening stones are passed over it vigorously. Be sure to check your surface often to make sure you do not take off too much material. Once your stones are true, you are ready to begin shaping the surface.
If you have a chipped or broken blade tip, you can reshape it using the side of your coarse stone. With the edge up, move the tool back and forth in a straight line, shaping the tip to a good point. This will groove the stone, which is why we use the edge instead of the surface. Once your point is properly shaped, it is time to proceed to sharpening the tool.
How your tools are shaped
Look closely at the blades of a grafting knife (or your better bonsai shears). If you have a new one, this will be the most instructive. You will find that these blades are not shaped like your household scissors or any other pair of pruning shears. Where most knife edges are formed where two angled planes meet, a grafting knife edge is formed where a flat (actually slightly concave) face meets a long, flat, beveled edge. Examine the photograph carefully. You will see a fine line about a quarter inch from the edge of the blade. This is not a beveled edge, it indicates where the layer of better steel is laminated to the body of the blade. The entire polished face of the blade is a single plane, honed to an edge a few molecules across.
Where other scissors or shears are designed with a definite bevel at the cutting edge (to make sharpening easier and to enable the edge to last longer), bonsai shears are designed so that the back face and front face meet at a very acute angle. Thus a single edge is formed, making possible the sharpest cutting edge possible. The most important point when sharpening a blade like this is to hone the entire front face of the blade, removing material evenly across the face, providing an edge that is ultimately sharper than a razor. But how does this information translate into actually forming that edge?
Making an edge
The key to forming a good edge on your flat-bladed tools is in your honing tools and the proper angle of attack, as it were. As you can see at the right, a shadow is formed if the edge of the blade is held too high. This angle will merely round off the back of the blade, and nothing will be accomplished. On the other hand, if the back of the blade is raised, the edge will form a bevel, which will never be sharp enough to slice the living tissue of the tree without damaging it. For the best results, the face of the blade must be kept in full contact with the sharpening stone throughout the sharpening process. The photo above shows the proper angle for a pair of bonsai shears, while below is incorrect.
As sharpening progresses, a slurry will form, made of material from the sharpening stone and the blade. It is important that this slurry remain on the stone to facilitate sharpening of the blade. As the moisture in the stone drops, it is crucial that it be kept wet. You can add fresh water, but it is easier to keep the slurry if you reuse the water that has drained from the stone, as it has some slurry already within it. Always keep your stone wet and your slurry in place.
The process, once you are comfortable with the position of the tool against the stone, is one of nearly mindless repetition. Many strokes are required to properly shape a dull blade. One with gouges in the edge may need grinding before it can be properly honed. Be careful of your fingertips. I have yet to cut myself on a blade, but I have worn off several fingernails and fingertips by carelessly rubbing them against the stone. As you make progress, you will want to move to finer stones until you are satisfied with the sharpness. Ultimately, the goal is to polish the face to a mirror surface. This will ensure the finest edge, making the smoothest cuts, helping the tree to heal itself quickly and easily.