Abstract: A process of ink jet printing on a variety of substrates and an apparatus for performing that process. In which process: a primer is applied to a substrate material; ink is ink jet printed onto the primed substrate; a characteristic relating to print quality is evaluated; the composition of the primer is adjusted in dependence on the evaluated characteristic relating to print quality; and the adjusted primer composition is applied to the substrate material and ink is ink jet printed onto the primed substrate material to give a printed product.