Faced with this problem when painting wallpaper: we apply water-based paint on the wallpaper (for painting) in several layers and, after drying, if you look at the wall directly, it seems that painted evenly, if a little at an angle, in some places as if painted in spots (a little darker, a little lighter).
The paint is not old, Dulux. Tried without primer, and with it, and the first coat of white paint. If anyone has dealt with it, please write in what may be the solution)
The solution may lie in a better preparation of the walls. The dark spots are not hollows in the walls?