How can I remove years of built up hard water stains in my bathtub?

http://www.rd.com/home/cleaning-organizing/6-household-items-for-a-sparkling-bathtub/

We have had success with CLR and scrub brush with firm bristles.

oven cleaner and the mister clean white sponges. I got this from pintrist. Make sure u have lots of ventilation. I used oven cleaner with low odor it is usually in a blue lid. Spray it on and shut the door In about an hour use the white sponges and scrub gently or more if needed. After u get to where u need use turtle wax to coat the shower mine felt like brand new.

i use toilet bowl cleaner the gel kind. let it sit for 15 min and then scrub with a kitchen scrubber. it take a few times bu things shine again