What characterizes a tack coat in painting?

A tack coat is characterized by a layer with a reduced film thickness. This type of coat is intentionally applied very thinly to create a level of tackiness on a surface, which aids in the adhesion of subsequent layers of paint or coating. The purpose of a tack coat is to promote bonding between layers and ensure that they adhere well together, preventing issues such as peeling or flaking in the future.

Unlike a full layer that aims for complete coverage, a tack coat works by not fully obscuring the underlying surface. It is specifically designed to remain slightly sticky to enhance the connection with the next layer applied. Tack coats are especially important in coating systems where layers must bond effectively for the overall durability and performance of the finish.

The other options do not align with the characteristics or purpose of a tack coat. A primer for underwater surfaces or an undercoat for textile applications does not relate to the use of reduced film thickness for promoting adhesion between paint layers, which is the essence of a tack coat.