When painting metal, it's important that you take precautions to prevent mistakes in applying the paint. Whether you're painting a small, pewter figurine or spray-painting your mailbox to make it stand out a little more, use the right sort of paint for any metal surface.
Maybe the most important coat you can put on metal paint is the primer coat. If you apply a coat of metal primer, you've sealed in the metal and protected it from the elements. Over the primer you could use practically any kind of paint you want, as long as it's rated for the outside and exposure. In this case you're not painting the metal, you're instead painting the primer.
If you're going to be painting directly onto metal, it's important that metal is clean and that all impurities have been sanded and washed off. Once that's done, you can use metal spray paint to coat the metal. Metal spray paint is specifically formulated to bond and hold with a metal surface.
If you're doing more detailed work, such as painting metal models or miniatures, you might not be able to use spray paint because of the area it effects. However, there is metal paint that you can brush on. If you choose to use this type of paint, the brush must be clean of any impurities, and you can't use it for any other type of paint.
An option that any paint project, including those that are using metal, is sealer. Similar to the base coat, sealer provides an additional layer of protection over the layers of paint that you put on your project. Sealer can be sprayed or brushed on, and it's important that you use the proper type of sealer for a metal painting project. The types of paint it works with will be listed on the side of the can.