My husband rescued an OLD cast iron farm sink, but it's stained badly. I don't need it to be brand new white...I'm going to use it in my almost done, green house!! Thanks for any help!
I would try a liquid something-mild to start with such as Simple Green, which will clean & get major dirt off without harming the porcelain finish. Then you can move on to Bar Keepers Friend which is a natural scouring powder. It doesn't have bleach (like Comet, Ajax, etc), so you will need to do some scrubbing, but no bleach is easier on the porcelain finish.
If you're interested, you can also make your own scouring powder. Check this link.
Here is a link that might be useful: Homemade scouring powder