Thank you, all
Painting on copper is quite uncomplicated.
First I fixed the copper plate to a fiberboard panel with a stabble across each corner of the copper plate.
Cleaned the surface with turpentine and tissue and then scumbled over the whole panel with undiluted paint using a bristle brush. This made a foundation suited for further application of paint.
Then some marks for drawing and impasto paint.
I once before tried to paint on copper, but thinned the paint too much. Thinning is a no no because the copper is non absorbent. That time I gave it up without a fight.
I don't think that oxydation will occure when the copper plate is varnished.
Rembrandt and others have painted small pictures on copper. They seems to be in perfect condition.