Where I live we have very hard water. Because of this hard water I have battled trying to keep my bathtubs clean. I might add it’s been a losing battle. The mineral deposits and soap scum accumulate faster than I can scrub it off. I tried all of the products and nothing worked. Our bathtubs are very old so they look bad very quickly.
I discovered recently that a plastic mesh scrubbie, like you use to wash dishes works perfectly to clean all the crusted deposits off. And it takes minimal muscle power. A little comet or baking soda, some water and some scrubbing and you have a nice shiny tub.
So, if you are having a hard time getting your tub clean, try a plastic dish cleaning scrubbie. It works for me!