Which of the following is NOT a cause of adhesion failures?

The proper surface preparation is indeed not a cause of adhesion failures. In fact, effective surface preparation is crucial to achieving good adhesion between the substrate and the coating. It involves cleaning the surface to remove any contaminants, such as grease, oil, dust, and other residues, which can inhibit the binding of the coating to the substrate. Moreover, surface preparation can include creating the appropriate surface profile, which is essential for the mechanical interlocking of the coating with the substrate.

When surfaces are prepared properly, the coating has a greater chance of adhering effectively, leading to a durable finish. This contrasts sharply with the other options, where contaminants, exceeding the window for applying a topcoat, and insufficient surface profile directly contribute to adhesion problems. Thus, proper surface preparation sets the stage for successful coating applications, making it a critical component in preventing adhesion failures.