I have two cast iron bathtubs. One has been well taken care of. No abrasive polish was used on it. I imagine that grandma took her towel and wiped the tub dry when she was finished. The tub is about 90 years old and still has a shiny glass finish on it. It looks almost new.
The other tub was scrubbed to within an inch of its life, as they say. the glass finish is worn off. The surface is porous, picks up dirt and stains.
I am forced to use bleach and commercial products on tub number two and still it doesn't look so hot. It makes me think of teeth care.