I've also found that drain cleaner cleans a lot of nasties from the tub.
If you get the gel type, you can pour it on the stain and the gel will
stay in contact with it longer. Also, a lot of people have had decent
results with those imitation "Mr Clean Eraser" things.

John wrote:
I have a white steel tub that was brown, brown when we moved in
recently. Today I tried to get rid of the stain and was 80% effective
just using plain vinegar (4.2%). It just dissolved away so I assumed
that a product like CLR and Lime would finish the job. No such luck.
Has anyone had success with easily available products that will finish
my job and get me out of installing a new tub! Thanks.