Apparatus for painting mainly flat products including a spray booth, in its turn including a plenum, a conveying belt for conveying products at least a device for applying paint, and at least one suction plant, wherein the plenum of the spray booth has a symmetric M-shape including, from the respective peripheries to the centre of the plenum:



two first descending surface portions tilted towards the periphery;
two second lateral surface portions parallel to the belt;
two third descending surface portions, with a tilt towards the centre, opposite the tilt of the first portions;
a fourth central surface portion parallel to said belt, the fourth central surface portion being closer to the conveying belt than the lateral parallel sections.