An undercoat, also known as a primer coat, is a layer of paint that is applied to a surface before the final coat of paint. It is commonly used by professional painters to prepare a surface for a smooth and even application of the topcoat. Typically, an undercoat is applied after a separate primer that prepares the surface for painting. Undercoats may have a neutral color and are often used on metal and wood substrates. In addition to providing a smooth surface for the topcoat, undercoats also offer an extra layer of protection against moisture, rust, mold, and rot. They can also act as a barrier against damage such as dents, knocks, and scratches. Overall, using an undercoat is an essential step to ensure a high-quality and long-lasting paint job.