Punch a hole in your copper blanks and sand the entire top surface of the blanks using fine sandpaper. You want the gesso to adhere to the metal which is why sanding is really important. Use fine sand paper or fine steel wool to sand the surface. Be sure to sand any area you wish to color on.
Clean the blanks using Bon Ami cleanser and don't touch the surface with your fingers or you may transfer oil or dirt onto the surface and prevent the gesso from adhering properly.