Here is a sample of an unprepared piece of copper. You can see the various scratches and marks across the surface, which if left as is, would be unappealing on a finished painting. My aesthetic sense tells me that the purpose of painting on a shimmering panel of metal is to let the beauty of that metal glow through. Some artists like the metal for the smoothness of the surface and cover the entire surface with paint. Well, I like that too, but the glow is the most important to me. Therefore, I want the surface to be nearly flawless before I begin painting, so that I can leave holidays in the surface here and there.
The first step is to roughen the surface not only to remove the flaws but to give the paint something to adhere to. I used a wet/dry silicon carbide 1000 sandpaper and a hand-held electric sander, going across one direction then the other several times trying to remove any marks made by the sander. There will be some, but they will disappear after the first layer of paint is applied. You can see how the sheen is knocked down some and there is a minimum of streaking.
