Quick and easy power sharpening
You can save time and energy by using a rotary tool to sharpen the cutters. The Dremel A679-02 (available through our affiliation with amazon.com) comes with three grinding wheels in popular diameters and a guide that quickly screws onto the rotary tool to control the cutting depth and angle (it also comes with a stone and guide for sharpening lawn mower blades). Be sure to wear safety glasses while using it.
Place the grinding wheel against the cutter with the edge of the metal guide parallel to the cutter's factory-ground angle, turn on the tool, and grind the cutter with sideways pressure to the count of four. Sharpen every other cutter as you would by hand with a round file, then switch and do the opposite cutters. When you get used to sharpening with a rotary tool, you can leave it running as you move from cutter to cutter to make sharpening go even faster. The Dremel manual explains how to lower the depth-gauge fins with the grinding wheel.