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In a multi-coat plaster job, the first coat applied is known as the scratch coat. This initial layer serves several important purposes. It provides a strong foundation for the subsequent layers and is designed to create a rough surface that allows the next coat (the brown coat) to adhere properly.

The scratch coat is typically applied over a lath (a framework that provides support) and is scraped or "scratched" to create grooves. These grooves increase the surface area, improving the mechanical bond between the scratch coat and the brown coat that follows. This enhanced adhesion is crucial for the durability and longevity of the plastering system.

The other options refer to different stages of the plastering process. The finishing coat is the final layer that provides a smooth and aesthetically pleasing surface, while the brown coat is applied after the scratch coat and further prepares the surface for the finishing coat. The term base coat is more general and could refer to any foundational layer but is not the specific term used for the first coat applied in this context. Therefore, identifying the scratch coat as the first layer underscores its critical role in establishing a secure and effective multi-coat plaster system.