The important part of metal surface preparation is that all flaking paint and corrosion must be removed with a wire brush, scraper or sand paper prior to painting. Again, it is vital to undertake an Adhesion Test to make sure that your substrate is sound and will bond well to your paint.
If the surface is in a fair state, it may only need a gentle sand to assist the fresh paint to adhere to it.
It might be most effective if you are not certain of the surfaces condition, to utilise a small grinder with a fine sanding disc to run over the surface. Once back to a solid base you can now consider painting.
On metal areas, use oil primers and undercoats. The use of water based or acrylic paints can encourage rust.