Place the brush cutter blade on a work bench. Clean off the entire blade with a wire brush and soapy water. This will remove any dirt and debris from the blade prior to sharpening.
Dry the blade off with a rag. Rub the blade with lubricating oil using a clean rag.
Hold the cutter on the edge of the bench with the angled portion of the blade facing out. Position a medium-grain whetstone on the surface of the angled cutting portion of the blade. Make sure the stone is at the same angle as the blade edge. Drag the stone across the blade in even strokes until you are satisfied with the edge you have achieved.
Hone the edge of the cutter with a fine-grain whetstone until the blade is sharp. Use the same technique as with the medium-grain whetstone.
Apply a light coat of lubricating oil to the cutter with a rag.