Prep is the key to any good, long lasting, paint
job. And how you prep depends on the material. If it is aluminum
, most recommend priming it with special self-etching aluminum
primer. Zinc chromate is the old standby but I think there are newer ones out that may be as good or better (and I am sure some worse). But you totally need to get rid of any "loose" old paint. Bead blasting is very good for that but you may not be able to get that done.
And you need to sand/scuff any paint and all the metal before priming and between coats if recommended by the paint manufacturer. As far as which paint to use I can't recommend that, but two part paint is more durable. You could send it out for powder coating. You can't powder coat in place as far as I know.
You might go to a rigging
shop which repaints masts/spars. I think they mostly either powder coat or use 2-part paints.
Cleanliness at all stages is absolutely required - both for resulting durability and also looks.