I have tried vinegar and baking soda.
Bleach, Efferdent tabs in water and put on the spot depending on where it is. Reg comet Wet sprinkle and let set a while.
, CLR. for a few.
It is going to be really tough project. Try that pin https://www.pinterest.com/pin/538250592961516654/?nic=1 You maybe successful but I would consider re-glazing that you can also do it yourself in the spots. That is of course a quick fix that may not last long
Try Bon Ami or Bar Keepers Friend.
A lot depends on the type of Finish your tub is? Iron base, Porcelain, Acrylic?