Primers fulfil several functions simultaneously in the painting trade. They equalise varying absorbency levels of a substrate so that the topcoat dries evenly and no patchy areas develop. Deep-penetrating primers penetrate porous or friable substrates and consolidate them. Adhesion primers improve the bond between difficult substrate materials such as glass, metal, or smooth plastics and the subsequent coat. Barrier primers prevent the bleed-through of nicotine stains, water damage, or tannins. The choice of correct primer depends on substrate type, service conditions, and the topcoat to be applied. A correctly executed primer coat significantly extends the service life of the entire coating system.