That old wrought iron fence or steel gutter may look terrible, but with the right preparation and primer, most rusty metal surfaces can be restored to almost new.
The first step is to clean off loose rust and flaking paint and then apply a rust-inhibitive primer. You don't need to get down to bare, shiny metal—just clean off the flakes and powdery surface rust that prevent paint from adhering. However, badly rusted areas need more prep work and possibly a special primer.
Start with a wire brush (Photo 1). If the area is heavily rusted and pitted and wire brushing by hand is ineffective, use a drill with a wire wheel (Photo 2) and a primer that adheres to rusted surfaces, such as Krylon Rust Tough or Rust-Oleum.
For spots that are very badly rusted or too delicate or detailed to clean thoroughly, use a product like Rust-Oleum Rust Reformer, which soaks into the rust and changes it into a non-rusting, paintable surface (Photo 3).