I wouldn't use any thing not edible such as WD40, motor oil, etc. If there is rust there I would try to clean it up a little bit with some steel wool or a very fine sand paper. Tape off the area around it if using sand paper so you don't scratch the good area. Then season it like you would a cast iron skillet a few times as you cook on it. If the grill sets out in the weather a lot, wouldn't hurt to wipe the spot occasionally with Veg Oil or something.