Our cast iron bathtub is 50 years old and I have no plans of parting with it.
When the water drains out of the tub, there is a spot near the drain where a little water settles and over the years, a brown circle developed in that area and I could not scrub it off. It was hard calcium buildup.
I was cleaning out some drawers and found a pumice stone and decided to give it a try. I started to work on the tub and removed some of the buildup and then decided to let it soak in a paste of Comet cleanser first.
With a lot of rubbing with the pumice stone and the wet Comet powder, it came up and the tub looks so much better. It did not scratch the surface at all. From now on, I will dry the tub after it's been used and not leave it wet. I think that will solve the problem of calcium buildup.
Using the same method, I also removed the ring in the toilet which was also calcium buildup. There is not much I can do to stop that from happening so this pumice stone will go in the bathroom cleaning kit.
I read in a Heloise column years ago that Comet scratches the surface and you will always have embedded dirt. Since your tub is 50 years old, Im sure the previous owners used it. Heloise said to use liquid dishwashing detergent and a nylon scrubbie. Since I had my bathrooms redone over 10 years ago I have used only that and a Magic Eraser. I also use it on the counters. My tub and counters are in terrific shape.