On brass, nickel, lead, tin, aluminum and other bright-finish metals, a nice effect can be produced by painting the transparent color directly over the metal. This method will give a more subdued appearance to the finish while adding some of the reflective characteristics of the original metal. As for primers, well, that is up to you. You know your product material better than we do. While we can tell you that we use this or that, and that this other thing has not worked for us at all, you may have tried those same things and found that they work well for your application.
Experimentation is the key.
Good results have been obtained using airbrushes, touchup guns and various other types of spray equipment. As with the other items referred to in this note, your results will depend on your application and means of use. Experimentation seems to be the best teacher. Remember to keep the paint agitated, as the glitter will settle to the bottom of the cup. Drying times will vary also.