In which scenario is it crucial to avoid exceeding the topcoat window during coating application?

Exceeding the topcoat window during coating application is crucial to avoid because it directly affects the adhesion quality of the paint or coating system. The topcoat window refers to the specified time frame during which a new layer of coating can be applied over an underlying layer, and adhering to this window is critical for ensuring that the coatings will chemically bond properly. If a topcoat is applied after the window has closed, the underlying layer may have cured too much for optimal adhesion, leading to potential delamination, peeling, or reduced durability of the coating system later on.

Achieving a decorative finish, minimizing drying time, and saving costs are important aspects of coating applications as well, but they are secondary to the fundamental requirement of maintaining good adhesion. Without proper adhesion, any aesthetic or time-saving benefits may be rendered useless if the coating fails prematurely.