Can you clear coat over primer and not use a color coat when painting a vehicle?

Though it is possible to get clear coat to adhere to primer, there are a couple of reasons this is a bad idea. The first and foremost reason is that the primers main job is corrosion protection, coupled with a chemical and mechanical foundation coat for basecoat or color coat in a single stage system, it is not designed to be clear coated. The chemical bond between primer and clear coat would be very weak. Base coat paints have modifications in the resins to allow them to adhere to the primers, and still give the clear coat the proper foundation for it to adhere to, while flowing out in the wet look finish it is designed to have. Also, most of the primer jobs you see running around will have sun faded areas, as well as bleaching in short order. If you want the flat primer look, but have a job that will last, prime the vehicle, base coat it in the gray color you want, but clear coat it with one of the prepackage flat clear coats that are available. The resulting job looks like it is "just primer" but has all the chemical and physical protection of a traditional clear coat job, just not the gloss.