M.B.
Long ago, I used to use Whink Rust stain remover.... it worked wonderfully!
I don't know if it is still available in stores or not, but here is a link to the manufacturer's website... I think there is a store locator there, also.
https://www.whink.com/index.cfm?fuseaction=cEcommerce.Pro...
It would be in the laundry area at the store... try the big box stores to see if you can find it.