I tried one of the 'Tubby' kits on a cast iron bath. The coating lasted less than a year and flaked badly - I wouldn't recommend it at all. It's also very sticky and difficult to brush on and very tedious to sand the whole bath before application. If the bath is nice looking (roll top or otherwise cosmetically interesting), get it re-enamelled professionally, otherwise it's probably best to replace it. Mine was 1/2" cast iron and very heavy so it was a pity to scrap it, but it was very ordinary so c'est la vie...
I hope this message posts - I posted this yesterday and it claimed to have done it, but it is/was not there.
If it's a bath you use occasionally then Renubath type in-situ recoating can prolong the life of a bath, and it can be re-done - but it doesn't last forever and in the long tern either proper re-enamelling (off site, hot oven type) or a new bath is more economic.
There are some in-situ coatings which cannot easily be removed - if you go that route, avoid them.
Renubath uses an epoxy type which I think can be removed with Nitromors.