What is the purpose of using a primer before painting aircraft surfaces?

Using a primer before painting aircraft surfaces primarily serves to enhance adhesion and prevent corrosion. The primer acts as a bonding layer between the metal surface of the aircraft and the topcoat of paint. This bonding is crucial because it helps ensure that the paint adheres properly. A good primer also provides a protective barrier that prevents moisture and other corrosive elements from reaching the metal, which is essential for maintaining the integrity and longevity of the aircraft structure.

The other options suggest benefits that do not accurately represent the role of a primer. For instance, while speeding up the painting process or increasing the thickness of the paint layer could be considered in some contexts, they do not capture the fundamental purpose of a primer. Additionally, while a primer can alter the appearance of a surface to some extent, changing the color is not its primary function. Thus, the correct answer focuses on the essential protective and adhesive characteristics that a primer provides in aviation applications.