Any tool with a cutting edge, whether old or new, must be sharp to perform satisfactorily. Mass-produced items such as chisels, planes, knives, lawnmowers, etc., generally need to be sharpened before use. Fortunately, there are devices and kits available for sharpening these tools; however, the trick is to determine which one is needed. Let's look at the requirements of lawnmowers.
 

What to use to sharpen your lawnmower blades depends largely on the type of mower you have and the condition of its blades. Most reel mowers, the ones that run on your energy, have five blades that are twisted from one end to the other so at least two blades are cutting at one time. Reel mowers can be sharpened with kits that contain lapping compound or abrasive strips. Rotary mowers, the types that operate on gas or electricity, can be sharpened with a grinding wheel, sanding belt, stone or a file.

For reel blades that are undamaged and just need their edges "refreshed" to bring them back into perfect condition, a kit with lapping compound is all you need. These easy-to-use kits usually include an application brush and a crank handle.

Remove a wheel (it doesn't matter which one) and the pinion gear, which is on the end of the reel shaft. Mount the crank handle onto the reel shaft. Use the brush to apply the abrasive compound along the entire length of each blade edge as well as on the cutting edge of the bar that makes contact with the blades. Turn the blades away from the cutting bar to sharpen them. You will hear a gritty, grating sound and feel some resistance, but this is normal. When the blades are sharp (after about 100 to 150 turns), you will feel a fine wire edge forming on the front edge of the blades. You may need to reapply the abrasive compound and repeat the sequence two or three times. Remove the remaining compound by spraying the blades with water, then wipe the blades dry.

To test blade sharpness, tear some newspaper into 2" wide strips about 12" long. Turn the crank so the blades come toward the cutting bar, and feed the newspaper strips between the blades and the bar so the newspaper is cut off across the 2" width. Each blade should cut the paper cleanly where it meets the bar. If the newspaper folds over, the blade may need more sharpening (check for burr on front of blade), or the cutting bar may just need to be adjusted. Remove the crank handle and reinstall the pinion gear and the wheel.

For reel blades that are nicked or badly uneven, kits with abrasive strips are your best alternative. Such kits come with a bar that holds an abrasive strip. The bar clamps to the cutting bar and the abrasive sharpens the blades at the correct angle as the blades are turned. To use this kind of kit, adjust the cutting bar to allow enough room for the installation of the bar with the abrasive strip on it. Position it on top of the cutting bar and readjust the cutting bar so that the blade comes into contact with the abrasive strip. Rotate the reel to sharpen the blade from one end to the other, until each blade is shiny from the abrasive strip and all nicks have been removed. Loosen the cutting bar, remove the bar with the abrasive strip on it and readjust.

You can sharpen the blade of a rotary mower on a bench grinder, belt sander/grinder or with a rotary blade sharpener that fits in your drill. You could also use a file. Before sharpening a rotary mower blade, disconnect the spark plug wire on a gasoline engine or the power cord on an electric mower. Remove the blade from the mower and remove the grass from the underside of the mower as well as from the blade. Grind the bevel back until you feel a burr develop on the back side. Remove the burr with a fine file, keeping the bottom flat.

If you must put a small bevel on the bottom of the blade to repair some damage, do it to both ends in order to keep the blade balanced (so your mower doesn't get the shakes and wear out your crankshaft bearings and your nerves). To check your blade's balance, put a narrow edge tool (such as a putty knife) vertical in a vise, and set the mower blade on top at the center point. If the blade consistently leans to one side, remove more metal from that side (regrind that bevel). When the blade is balanced, re-install it on the mower (with the bevel up and the flat side down), set the mower upright, and reconnect the spark plug wire or extension cord.

Resharpen your lawnmower blades as frequently as your grass gives you signs that your mower blades are getting dull. If the grass tips turn brown or the grass looks torn rather than sheared, it's probably because your mower blades need to be resharpened