Things You'll Need
Automotive Valve-grinding Compound
One of the beauties of a push lawn mower is that it sharpens itself to some extent: As you push the mower, the revolving blades scrape over the cutter bar, and the action tends to make both blades keener, not duller. Every year or two, though, a sharpening job is in order. You could take the machine to a pro, or use this backlapping technique.
Prop up the lawn mower so you can turn the reel by twirling the wheels.
Check the blades for nicks and burrs. If you find any, remove them by holding a file flat against the blade and pushing away from the edge.
Examine the cutting bar. It should just meet the blades along their entire length. (To make sure it's in the right spot, grasp a wheel and turn it forward; you should hear a whispery sound as the blades pass the bar.)
If the bar is out of alignment, adjust it using the screws on the ends of the bar. (There are two at each end; when you look at them, it will be obvious which one will move the bar closer to the blades.)
Using your fingers or a soft paintbrush, cover the blades with a thin, even coat of automotive valve-grinding compound (available at auto-parts stores).
Grasp a wheel and turn it slowly backward so that the grinding compound is squeezed between the blades and the cutting bar, thereby sharpening both cutting edges as they pass each other. Be sure that each blade touches the bar as you rotate the wheel.
Make about a dozen turns, applying more grinding compound when necessary.
Examine the blade edges. If they look sharp, wash off all the valve compound with soapy water and rinse thoroughly.
Test for sharpness. Insert a sheet of newspaper between the blades and the cutting bar, and rotate the reel forward. (You may need to try a few times to get the paper in the right position.) When the blades cut the paper as easily and as cleanly as a sharp pair of scissors would, you're ready to roll.
After each use, hose the mower clean and wipe it dry. Then wipe all the metal parts with a thin film of WD-40 or mineral oil. If you've got a new push mower, check your owner's manual for any specific sharpening guidelines.
Sharpen blades only by hand. The heat generated by a power grinder or an electric drill with a grinding wheel attached can destroy the temper of the metal.