The most reliable way to sharpen planar knives, usually called planer knives, is to send them out to be professionally sharpened. Professionals have years of practice and can obtain a nice edge without sacrificing too much blade. With a little practice and the right tools, you can maintain a sharp planer knife yourself, though you should still send knives out annually to be sharpened.
Place fast-running grinder on a flat location with ample light, and plug the grinder into an outlet. Before using, always make sure the wet wheel is we,t and moisten with a damp cloth if necessary.
Hold the planar knife blade against the grinding wheel at a 20-degree angle. Make sure the length of the blade can be sharpened, and change your position if necessary, making sure to maintain the same angle so the blade will be uniform.
Hold the blade up to check whether it needs further sharpening. If you hold a blade straight up, a sharp blade should not reflect light while a dull blade should show a gleam of light along its edge.
Continue sharpening until the blade passes the light test outlined in step 3.
Make sure the blade is straight by placing it against a flat surface. If the blade does not lay straight, sharpen any angular areas before using.