I really like products that are dual purpose. I have a fiberglass insert tub (so I'm not sure if this would work on iron tubs). On a day when i go to the gym,so the tub is totally dry, i take about 1/8 cup liquid laundry detergent (right now, I like ERA) and use my scrub poof (like you get with Dove liquid cleanser) to rub the detergent all around the tub. When i get back that night, the detergent has dissolved the hardwater scum and it's real easy to scrub clean. I should probably do this every week, but I'm lucky if I get to it once/month. I used to buy scrubbing bubbles which works great and FAST, but not anymore. I also use my liquid laundry detergent as a pre-treater.

I've tried the spray to use after getting out of the tub, supposedly so the tub will stay clean, but it never worked. If i had a water softner, I'm sure it would help.