The flat black look was popular on "rat rods" in the late 1960s. Rat rods were built to be functional, not for show, like many hot rods of that era. This often meant using black primer as a finish instead of glossy paint. That flat black look is back on the custom car show scene, but the colour still looks most appropriate on classic cars. Another advantage of using primer is that it hides imperfections and wavy sheet metal, instead of showcasing them like glossy paint does.
Sand the entire car with 280-grit sandpaper until the top layers of existing paint are removed and the car's surface is dull. Use a sanding block, a dual-action sander or folded sandpaper sheets. It's OK for the surface to have different layers of paint showing at this point.
Wipe the car's surface with wax-and-grease remover and a cotton towel. This removes any fingerprints, grease or wax contaminants that could cause bubbles or other imperfections in the finished primer surface.
Place automotive masking tape over small areas like the door handles, key holes, lights, weather stripping and trim. Use paper and tape to cover larger areas like the window glass, grille, tires and rims.
Pour the black primer into the can of the paint gun and attach the air hose from the compressor. Adjust the pressure and spray a test surface, like a piece of cardboard, to make sure the gun is spraying evenly.
Spray the primer, starting with the roof of the car, moving from the front of the car towards the rear. Overlap each stroke several inches for full coverage. Move to the bonnet, front fenders, doors, boot lid, rear quarter panels and then the rear of the car. Let the coat of primer dry for 30 minutes and apply a second coat. Repeat this for the third coat. Let the final coat dry for three hours.
For a longer-lasting flat black look, have an automotive paint store mix a flattening agent into black paint to make the paint look like the flat primer. The paint will last years longer than primer.
Never spray primer without wearing a painter's respirator mask.