Place the end of a tube of graphite powder against the exterior lock at the key opening. Squeeze a small amount of graphite powder into the lock. Insert and remove the key repeatedly from the lock. Insert the key and repeatedly turn the lock back and forth, locking and unlocking the door. If the key and the lock still stick, add more graphite. Do not spray any penetrating lubricant into any part of the sliding door lock if the lock continues to stick. This will only gum up the lock. If the lock still fails to open and close smoothly and the key continues to get stuck, replace the cylinder and latch.