Crud exists to keep tub-reglazing specialists busy and profitable.

Our house is almost a century old (built in 1914), and the cast-iron porcelain-glazed clawfoot bathtub almost certainly dates back to 1914 as well. By the turn of the most-recent century, so much of the porcelain glazing had worn off that the inner surface of the tub had become porous and easily prone to staining. My wife eventually banned the use of all bath salts except plain-white epsom salts, because the other fancier hippy-trippy aromatherapeutic stuff was leaving a nasty-looking green residue that couldn't be scrubbed off.

Finally, quite recently, we had the tub reglazed. Now it's shiny and bright white, and much more easily cleaned. The only drawback is that it's much more slippery than before when used as a shower, but we're adjusting.

The deterioration of the tub's inner finish can only be blamed on crud ~ either because crud not quickly cleaned off has had a destructive effect over the years, or because the very act of scrubbing the crud away has gradually worn away the surface.

Actually, BOTH have probably contributed to the gradual destruction of the porcelain: occasionally leaving crud in place too long, and also the application of elbow grease to remove the crud, whether in a timely manner or after sitting for a whle. Can't win for losing...