The problem with cast iron is that it is very pourous. Once salt
has pregnated into the iron it's there to stay. If it's pitted, you'll never get that stuff out. But you can seal it up!
If you want to smooth it out for perforance. You can fill and fare it with the West Systems epoxy and fillers or any other epoxy, for that matter.
Any pits you may want to sand blast to get as clean as possible. Then make sure they are real dry by using a propane
Once the whole keel is clean and dry and at 70 degrees or above you'll want to get a coat of epoxy on there right away so the moisture doesn't absorb back into the iron. Once you get that first coat brushed/rolled on you can take your time doing the fill and faring work at your leasure.
The main thing is to get it sealed without moisture!