How do I get rid of rust on an old tin bank?

http://www.oldgas.com/shoptalk/00a/messages/1352.html

On the link above you will find someone's recommendations for cleaning
old tin oil cans, which I imagine would work equally well on a tin
bank.  They recommend using pure citric acid in water and letting the
item soak.

Hope this helps.

If, by "old tin bank" you really mean "potentially valuable old tin
bank", DO NOT CLEAN IT NO MATTER HOW RUSTY IT IS!

Rust, tarnish, and wear and tear can sometimes be deceiving. Many
antiques can drop in value as much as 100% just because someone
cleaned them.

tutuzdad-ga

tutuzdad-ga is absolutely right, of course.  Antiques are almost
always best left alone, or professionally restored (at the advice of
an appraiser).  However, if you aren't keeping the bank as an antique,
it would be a good idea to stop the rust from getting any worse and
destroying your item.