The impregnation coating process, also called saturation coating, is used to coat porous substrates that have a substrate structure consisting of pores that penetrate the entire substrate. This process coats the internal web structure and fills the pores with a variety of thermoset polymeric resins that reinforce the substrate. The pores can be filled with no surface coating or they can be filled and have a surface coating. The resulting modified substrate is solid, has a surface with different characteristics than the uncoated substrate. to keep out chlorides is the most common reason for applying these coatings.