Careful about using terms that define special relationships like highest and lowest, use chronological terms like earliest and latest. I think it is the attempt to,directly infer spatial relationships that gets people confused. The canvas would be the earliest context on a painting to be followed by the materials used to prepare the surface then the various layers of paint in the order they were painted. The canvas could be upside down, on the floor or suspended on the ceiling when painted but that would make no difference to the matrix.
Unless the wall floor or ceiling is the surface the painting is on but even then the relationship is not physical,it is temporal (before, same time as, after).
Unless the wall floor or ceiling is the surface the painting is on but even then the relationship is not physical,it is temporal (before, same time as, after).