Calcium buildup

Persistence. It will work but you cannot expect it to be a wipe on solution.

Are you sure it’s calcium and not paint?

Have you tried a Mr. Clean magic eraser. Might work.

Calcium helps make you teeth and bones, so it can really harden. Vinegar softens calcium. But it may take awhile to dissolve it. It is also possible that what’s in your tub is plaster from washing out brushes and buckets from another job. Use a butter knife or spatula to chip away at the mineral deposit after soaking the bottom of the tub. ☺️

Have you tried letting the vinegar sit for some time (an hour or more)? I have used a "magic eraser" to help with that problem in my old tub; I just put it down and rub a section every time I shower.

Just asking because my mom emptied a paint tray with a bit of paint in it down my tub drain..left it white on the bottom Of the tub...Her excuse...that’s a bigger drain...

Single-edge razor blades are the only thing I've found that work without much effort. They're the thinnest scraper blade ever, and slightly flexible for curved areas, and will not damage porcelain. They work on calcium deposits, and should work on paint. If the paint was water-clean-up when wet then acetone will thin it if the razor blade can't get it off at first. When it's a thin film, let it dry, then do the razor blade again. Once the porcelain is smooth (and you can feel that it is) dry the shower or tub with a towel. A spray of vinegar+Dawn after showering or bathing should help keep the calcium deposits at bay, and if they come back use the razor blade again.

Just checking..my mom decided to rinse out a paint tray in my bath tub. Was a bugger getting the paint residue off the tub. It’s going to take a bit off time and persistence to get it gone. Have you tried the cleaning vinager? Or just the plane Apple cider type? The cleaning type has a higher percentage of acid.