If you want to paint new, ferrous metal (e.g. steel, cast iron). Remove all traces of grease and oil using white spirit or methylated spirits.. Clean the surface with wire brush and brush off. Apply a suitable metal primer. Lightly rub down with glasspaper and brush off. Then apply one undercoat. and two gloss coats.
If it is rusted ferrous metal, remove all rust with a wire brush and rub down. Apply a suitable anti-rust treatment. Apply suitable metal primer. Lightly rub down with glasspaper and brush off.
Then apply one undercoat and two gloss coats.
There are also metal paints on the market that give a decorative finish.
Good luck