It is shown that, when a dielectric coating is deposited, its profile deviates from the initial profile of the metallic grating ("blurring" occurs). The amount of blurring is determined by numerical modelling and by measurements on an electron microscope. It is shown that the decrease of the depth of the rulings is the main factor that reduces the diffraction efficiency and the radiation strength of metallic diffraction gratings with multilayer dielectric coatings. The blurring effect can be levelled off by optimizing the deposition conditions of such coatings and by using dielectrics with a large refractive- index difference.