I recently used a Rustoleum tub and tile kit on my kitchen sink and although smelly and very specific steps to get a nice finish, it was worth the time and effort it took. If you want to just clean it for now, try Bon Ami or Bartender's friend to scrub it out.
The enamel physically wears out and becomes more porous.
I would use a calcium lime rust remover.
Use a magic eraser after using Barkeepers friend.
I spray painted the inside of my claw foot tub (which sits outside on the back deck) with white enamel spray paint...worked fine.