Please listen to Sherry 1. Do not attempt it. It can not be done.
All you will do is waste time and money and it will end up looking a lot worse than it does now.
Enamel in a powder form is fused on when the metal bath is pulled out of the furnace and is still red hot. Once the bath is made the process can not be duplicated.
Many firms advertise re surfacing baths but you will never get the same finish as a new one..
Go on, invest a few bob, a metal bath will last a long time if it's looked after. And you won't regret it.