As Svar says, if you want to highlight surface imperfections, you want the light source as close to the surface as possible, so that even tiny defects cast a disproportionately large shadow. Inspecting the surface under ‘normal’ viewing conditions (from a distance, with distant natural or domestic lighting) is the last thing you want to do, because as soon as the customer stands close to their wall, or runs their hand over it, they’ll notice all the defects you missed.