Black is the hardest color to paint a car. If the body work is not perfect you will see every wave, dip, wrinkle. If your car has seen many paint jobs over the years (stacked up) , you will need to strip it. If there are only one or two layers, and you know that the work under it is good, you could spray a build up primer, and start block sanding. Then guide coat, then block sand, and continue until it's straight. Then it's paint time. Are you planing on doing this yourself?