My Nana asked me if I knew of anything to take off the stain. She's tried Tilex, a commercial cleaner recommended by the realtor, and a pumice like stone designed for cleaning tubs naturely. She's been working on the tub for a while, but has only seen some improvement. She's wanting to try a few more things before she gives up.

I've been searching the net, and will consult books in the library. I would appreciate any suggestions.

So far, I've found these options:

Like the poster above said, vinegar and vinegar solutions often came up in my searches. Since this tub has a bad stain, I'm leaning towards the solution described here.
www.howtogetridofstuff.com/stain-removal/how-to-get-rid-of-hard-water- stains/

I remember a show that had bad cleaning problems, so I googled: queen of clean tub stain
I found a google book result that mentioned using powdered alum and lemon juice

I also found recommendations to buy professional descalers at hardware stores.

I don't know how well any of these will work on years of caked on hard water and soap scum stains in the tub. I'm looking for several things to try since I'll be there for a week.