I did this. It's not at all difficult but it is messy and hard work, but really, really worth it.
I used a product called peel-away. Its caustic material in the form of gunky thick
liquid.
You paint it onto the painted metal and stick on the supplied sheets of paper.
The next day, you peel away the paper and the gunk has dissolved away the
paint which comes away with the paint. You then need to wash away the gunk/paint.
The material is dangerous to the skin and will ruin the carpet/walls so do it before
you decorate. Cover every inch of skin and be prepared to use loads of water and old cloths.
Our fireplace had many layers of paint and intricit metal work so it took a number of
applications,
I have been honest about the work involved, but I hope I haven't put you off, the resulting cast iron fireplace looks fab and was worth the effort.