Still trying to figure out what "cat iron" is to be honest 🙂
But, IMHO, take the guttering down, clean it all up removing loose paint and rust with a good steel brush (maybe use something in a drill?).
Paint. Yes to a primer, then you top coat.
If you can afford it Farrow & Ball do downpipe paint (but I guess its for the guttering as well). Other alternatives - Rustoleum, FLAG, Bedec and Centrecoat