Try a pumice stone.

Is good but not too good for protection at all to me !

probably the only thing that will take care or your problem with the stain is to have the bathtub painted by a professional company that specializes in resurfacing bathtubs. More then likely the porcelain is worn and the tub will always be hard to keep clean.

Try lysol toilet bowl cleaner (just the regular kind not the kind with bleach) and a magic eraser. I've used this severe times with great success! :)

Try to find a product called Zud. It is in either powder form, or cream liquid. It may be found in Home Depot, Lowes, or a supermarket. This product removes even rust stains with little scrubbing effort.