Every 2 or 3 years, for the past 16/17 years, I have re-coated my cast iron bath with Hammerite (smooth). It doesn't appear to be recommended by advisers but I don't know why. It's not easy to get the application right - the first time, I followed the instructions on the tin, i.e. to apply thickly - disaster, the surface formed a skin whilst still wet underneath and wrinkled. I had to scrape it off. Since then, I have painted a thinner layer, leaving 24 hours between each coat (between 2 to 3 coats), keeping room well ventilated. If you paint too thin a layer, it will drag and give a rough surface. Because the bath needs to be cleaned, the surface gradually wears thin and, if it is white, the colour goes off-white, hence the need for new applications.
You may have solved your problem by now, but, if not, hope this helps.
19scar