I see several hundred new students in my classroom each year, all anxious to start chopping wood. But there's a little tough love waiting for them: I provide students with the least expensive, home-center chisels available for two reasons. First, any accidental damage is no big deal because they cost only a few bucks to replace. Second, and most important, these chisels have relatively soft steel blades, so they dull quickly. Why's that a good thing? Because frequent sharpening ingrains in their minds and muscle memory correct sharpening techniques, which are the same regardless of the chisel quality. And having a sharp chisel always trumps having an expensive chisel.
No matter what technique or mechanism you use to sharpen your chisels, get comfortable using it and return to it regularly to touch up your tools. I favor sharpening freehand on waterstones -- years of sharpening experience mean I don't need fancy machines or gadgets. And, if treated properly, the stones last for years. But if you prefer to use a honing guide to hold your chisels at the correct bevel angle, or maybe a motorized sharpener, by all means do it. They all work if you take the time to get familiar with them.
Always start out a new -- or, dare I say it, an abused -- chisel with a hollow grind on a benchtop grinder, as shown (photo). I use 36- and 60-grit wheels because I want to shape the steel quickly and avoid heating up the tools. Overheating, usually indicated by a blue hue of the steel, causes the cutting edge to dull more quickly. And I prefer a hollow-ground bevel because it's easier and quicker to sharpen and touch up on the stones. (With less steel dragging across the stone, there's less friction, and the bevel's two-point contact helps to maintain correct angle orientation on the face of the sharpening stone.)
Flatten the back
After making the hollow grind, lap the back face of the chisel to make it perfectly flat, as shown (photo). You only need to do this on the bottom two inches. Some chisel work won't require a flat back, but other tasks, such as mortising and paring, do; so just get in the habit of lapping. Start with a coarse stone (800 grit) and repeat on several finer grit stones. This only needs to be done again if the back goes out of flat or if you have used up the flat portion of the chisel blade.
Sharpen the cutting edge
Next, sharpen the cutting edge on stones or sandpaper grits. Begin with a coarse grit (800 waterstone, gray Arkansas oilstone, or 150 sandpaper) to define the bevel, creating two narrow, flat points of contact above and below the scooped-out hollow grind (photo). Progress through several finer grits, up to 15,000 stone or 1,000 sandpaper, to hone the bevel to a razor-sharp edge. I go a step further and burnish the bevel on brown butcher paper or grocery bag.
Although it's not necessary, I like to add a secondary microbevel, about 1° steeper than the main bevel, for an even finer edge. To do this, after you've finished the first bevel, go back to an 8,000-grit stone or 1,000-grit sandpaper, raise the chisel slightly off the bevel so only the tip touches, and gently give it a few strokes. If you are anticipating doing a lot of heavy chopping or are working with a particularly hard or abrasive timber, such as white oak or silica-rich teak, add a secondary bevel. The more obtuse microbevel will dramatically increase the edge-holding durability of the chisel in these situations.
The more you sharpen, the more you'll dish out the stones. Keeping the stones flat guarantees your chisels' edges remain flat. Use a flattening stone to remove the high spots. I probably do this more than I need to, but I'm in the habit of flattening every stone before I sharpen with it.