Scale in a tub, I’ve tried detergents, scrubs, etc. No success to date.
Try "Bar Keepers Friend"
Hi there, the worst calcium build up I've had in my tub and the ONLY thing I could manage to work with some fine grit sandpaper just very carefully used to remove the worst of it. It worked great.
It worked fine in the bottom of my fiberglass tub (there was already a rough texture there) but I don't know how it would work on enamel or porcelain
Steel wool pad, gently with Ajax or Comet powder cleaner, should work, you may need to make the powder into a paste and dab it on with a rag or an old toothbrush, to ket that paste sit overnight.
Easy Off No Fume Oven Cleaner in the BLUE can. Works on enamel, porcelain, and fiberglass.
Do you use a scrub of any kind to apply?
Spray the oven cleaner and let it liquefy. Sometimes may need some scrubbing. A brush or green kitchen scrubby should work. Do wear rubber gloves. Rinse well.
Used as recommended and was extremely helpful. I would say it removed 75% of the problem. Still had something like a shade at the bottom and lower sides of the tub. Then I tried with solution of vinegar and dish liquid detergent + green scrub. Now looks much better, would say 90% better including some nice shine.