Tamiya primer should work well on metal. When I paint metal, I use a Duplicolor primer designed for automotive work, since it is engineered specifically for metal. Whichever one you use, the key to success is surface preparation, perhaps even more so than plastic. Some sort of abrasive cleaning of the surface (fine sanding, etc) to remove oxidation & contaminants and provide some tooth, and a thorough cleaning with a solvent that will not leave residue (alcohol or lacquer thinner, as appropriate).