Carlos, thank you for responding.
About the primer.
I washed the surface with terpentine to get rid of any fingerprints and dust. The copper I used was new but had been exposed to air, light, dust and acid from the fingers, when I washed it it became like new.
I don't think that sanding is a good idea sinse the stripes from the sandpaper will show through thin areas. Linseed oil paint will adhere to copper if the copper is clean from the start.
I then scumbled over it with a stiff, right from the tube, oil paint of a similar color as the copper, or a little more greyish.
This is not a real priming, like someting that chemically grip the surface and then dry before further progress. It is just so that I have a foundation to paint into, wet into wet.
It is important to NOT thin the paint with TOO much turpentine because it will dissolve the paint already on the painting and make a slippery soup, because the copper is non absorbent. This is also a good thing sinse it is easy to wipe the paint off when you mess up details
But if you have to paint on it when the first layer is dry I guess that it will be like any other painting surface.