I just thought of this one, though I've not tried it myself. Anyone ever used Bar Keeper's Friend (powder) and water on their water pan? BKF is supposed to be safe for porcelain coated surfaces as well as enameled cast iron pots, so it seems like it could work. BKF may have to be applied and left on for a a minute or two to allow it to work Use gloves if you try it though, as it can be rough on skin. BKF's active ingredient is oxalic acid, which occurs naturally in spinach, cabbage, broccoli, rhubarb, as well as other fruits & vegetables.