If you have regular black LINE-X:
Clean it with automotive soap and water. Scrub it with a stiff brush. Use CLR cleaner (available at most Wallymarts and Homey Depots) if it's heavily soiled or has rust spots. Rinse thoroughly. Once clean and dry, apply some Just Once (justonce.com) or 303 Aerospace to restore some color. Now, I have heard of others using MopNGlow, carburetor cleaner, and other stuff with good results, but I've never tried those things.
If you have LINE-X Xtra:
Clean it with automotive soap and water. You probably won't have to scrub it.
