Before applying metal paint it is common to use a good quality primer or sealer to stop any future problem with corrosion. Some very good metallic paints that are available today have built in primer.
Paint for metal is necessary because without this protection metallic objects when exposed to a harsh environment will corrode and rust very quickly. For example, a metal roof that is on a home by the seaside will be exposed to the ravages of sea spray and will corrode very rapidly. Paint for metal has advanced rapidly in recent years however unless the surface is prepared properly the paint will not protect the metal that has been painted.
If a poorly prepared roof with flecks of rust is painted over with metal paint it will not take long for the rust to bubble to the surface, no matter how good the metal paint is.
To avoid rust problems getting out of hand on any surface it is therefore important to always put the effort in early by taking steps to remove it completely by either using a metal brush or scraper. If any holes need to be repaired that are caused by the decay, they can be filled in with a filler and sanded to a smooth surface if necessary.