Primers ensure that the adhesive can adhere properly to the substrate and are mainly used on the surface of porous materials. It works by making the surface more solid and preventing the adhesive from penetrating the substrate too far or too quickly. This allows the adhesive to stay on the surface of the substrate and achieve a good connection. However, not all primers have the same functions. There are, for example, primers that will prevent humidity from reaching the adhesive layer through the surface that may degrade the quality of the adhesive.