If you've got an overgrown lawn and nothing but a non-functioning push mower housed in the garage or shed, you've got a problem, but one with a relatively easy solution. Rotary lawn mowers work only as well as their blades. When the blades are dull and inefficient you need to sharpen them and bring them back to proper functionality. Once you've done so you will be back in business and ready to get that lawn looking well-manicured once again.
Drain the mower of all gasoline and disconnect the spark plugs. This is an important safety measure, as otherwise the mower could start when the blades are spun by hand or if the mower is jostled.
Remove the blades using penetrating oil (such as WD-40) and a socket wrench. For specific instructions concerning your particular make and model of mower, check the instruction manual. Wear leather gloves to avoid injury to your hands.
Secure the blades in a vice, one at a time, and sharpen with a grinder. Take small, light passes to begin with, particularly if you do not have experience with grinders. Try to remove as little metal as necessary and keep it even on both sides. Follow the natural contour of the blade as close to the initial factory form as possible.
Balance the blade using a balance tester. This will allow you to see if your blade is weighted to one side or the other. If this is the case, grind more off the heavier side until balanced.