Don't know about tinning, but I saw a cast iron base tank that had been
>electroless nickel plated and it certainly looked good.
I've been warned off electroless nickel for cast iron, on account of getting porosity problems around any carbon pits. If you want to do it, I was advised to go with a nickel flash coat, then electro copper plate it to get a solid plate over the whole piece (this is dead easy, you can use an acid process) and then nickel flash it again to get the visual effect you want.