This is a very common problem with metallic paints. The metal flake lies down in a way that makes it reflective, but the addition of an overcoat based on the same solvent causes the metal flake to move, resulting in a dull appearance. Option 1, get an acrylic (water-based) clear to paint over the Rustoleum enamel. It won't redisolve the Rustoleum. Two, use a buffable metal paint like Unoudid suggested. These apply a thin layer of soft metal particles to the finish. By buffing it lightly, the metal particles are smeared flat and form together for a highly reflective finish. I've worked for years with Testors Metalizer Buffable Aluminum. It's available at any decent hobby shop in either a spray can or ready-mixed for an airbrush. There is a compatible gloss coat to go with it. See the sticky on visors in my sig block on how to use it. If you can afford a more expensive product, I've discovered Liquid Chrome from Eastwood.com since I wrote that visor Tut. It is as close to a chrome finish as you can come with a spray on and buff finish--less buffing to get the super shiny finish. Also, no matter which way you go, you will lose a slight amount of luster when you add a clear coat to protect it.
Redshirt