A primer or undercoat is applied to mineral materials such as plaster or concrete to reduce the porosity of the
substrate and compensate for different absorbency different substrates.
In a painting by less ink is required.
There are solvent-based and water (solvent-free) primers. In sandendem underground (usually in very old plaster layers or poorly processed concrete) is the primer solvent-dependent, since they can penetrate deeper into the surface and so more can contribute to a strengthening material. Solvent-based primer should, if possible, be applied only in the outer region, since such residues still outgas noticeable after several weeks of the machined surfaces. Solvent-free primer can be applied on all absorbent substrates in interior and exterior applications.