A clear coat should help, probably a matt finish would be best. Acrylic would do the job, depending on what you intend to use for 'weathering', lines etc etc.
Being an automotive primer, it's probably either cellulose, or synthetic enamel, so a polyurethane or enamel clear coat should also work.
Alternatively, dirt, staining etc could be 'rubbed in', using pastels, chalks, dust etc.