Cementitious primer underlay is a type of flooring system that is applied to a prepared substrate as a base layer. It is made up of cement and other additives that create a smooth and level surface, preparing it for the application of a top coat. Cementitious primer underlay is ideal for leveling uneven surfaces and creating a strong and stable base for other flooring systems. It is also highly durable and can withstand heavy foot traffic and machinery. Overall, cementitious primer underlay is an essential component of many flooring systems and is critical in ensuring a high-quality and long-lasting finished product.