I don't know if this fits your problem, but this is from my Consumer Reports book on how to clean things: for stains on vitreous or porcelain enamel, apply a solution of 1 cup of chlorine bleach and 1 quart of water. Leave for only 15 seconds, then rinse. Clean heavy mineral deposits with warm water and vinegar. For stubborn stains on glazed enamel surfaces, add ammonia to calcium carbonate to make a paste, apply, leave for an hour, and rinse. If the surface is unglazed, sponge uncolored household scouring powder and water onto the area and leave for five minutes. Scrub with a stiff-bristled brush and rinse.

Good luck.