One of the easiest ways to know that the blades need sharpening is that they look nicked and dented. Another sign is that the lawn mower isn’t cutting properly. This means that the edges of mown grass look ripped rather than evenly sliced; this stress can make the lawn turn yellow or brown.

Removing the blades depends on the mower. First, make sure that the gas and oil caps are tight. Tip the mower back and away from the carburetor side, so the fuel doesn’t leak out. Some lawn mowers, such as tractor models, need to sit flat and allow you to remove the deck. With others, you can tilt the machine back with the help of a jack, giving you full access to the undercarriage.

Many mowers have a single nut fastener that needs to be removed with a ratchet before you can take out the blade. If the fastener is particularly stubborn, use some penetrating oil to loosen it up first. Once you’ve taken out the blade, remove all of the caked on grass and mud with a piece of steel wool. Cleaning is important, because you need to properly see the blade. Plus, if you’re using a sharpening tool with high RPMs, debris from previous mows will fly.

A bench grinder is a grinding machine that uses abrasive wheels to sharpen edges. When using this tool, you just need to hold your lawn mower blade against the spinning wheel and draw it across until it’s sharp. The key is to maintain the blades’ curved angle while grinding one side at a time. It’s a good idea to wear goggles here, as the friction produces sparks.

Using a hand file is labor intensive, but it allows sharpening at the proper contour, and doesn't require a separate machine. Apart from the standard file, you need a vise to hold the blade in place while sharpening. Before you start, make sure you're working the chiseled side and not the flat side. Maintain the same angle that’s on the blade and file on the downward stroke, not back and forth, as the latter can wear out the file too quickly.

Rotary tools, like a Dremel or a simple drill, have attachments that sharpen other tools, like chainsaw teeth. In addition to being portable, such tools are lightweight and maneuverable, so you can control it better to achieve a nicer edge. You can hold it in your hands and run the tool over the edge as many times as it takes.

Many people use angle grinders to cut metal, but its abrasive wheels make it an ideal choice for getting rid of those nicks and dents in mower blades. Place the blade in a vise and fit the angle grinder with the grinding stone disk. Turn on the grinder and at an angle, run the tool along the beveled cutting edge, taking care not to press too hard.

Sharpening at the right angle depends on the method. If you’re using a 10-inch file or a grindstone, working at a 45-degree angle helps you to keep the pressure even. A manual approach also helps you to feel how each stroke works and if you need to apply more pressure. For an angle grinder, the directions will tell you the optimum angle for use, which is between 20 and 30 degrees.

After sharpening, it’s a good idea to check the blade's balance, to ensure even weight distribution. Unbalanced blades rotating at thousands of RPMs cause vibrations that can stress the shaft and engine, reducing the useful life of your mower. To check balance, take a nail, and clamp it horizontally in a vise or tap it into the wall. Hang the blade on the nail from the middle hole. If it lists to one side, it means that side is heavy and needs more work.

If you have a non-tractor mower, it may be easier and less time-consuming to sharpen the mower without removing the blades. Along with the necessary work tools and safety gear, you need to disconnect all power sources and remove the spark plug, so there’s no chance that the machine will accidentally start. Empty the gas tank to reduce the chance of a leak or a potential fire. To prevent the blade from moving while sharpening, wedge a couple of 2x4 boards in key stabilizing spots.