Why are the fasteners, joints, and any abrupt edges in the airplane sealed?

Sealing fasteners, joints, and abrupt edges in an airplane is primarily done to make them flush and smooth. This is crucial because a smooth exterior helps to minimize drag, which can improve the aircraft's overall aerodynamic efficiency. When surfaces are smooth, air can flow over them more easily, reducing turbulence and drag that could otherwise affect the aircraft's performance.

Moreover, sealing these areas can also play a role in preventing water ingress, protecting the integrity of internal mechanisms and systems from environmental factors. A smooth surface can also help in reducing the chances of accumulation of ice or debris, which could escalate maintenance issues.

While other factors like wear and tear, visual appearance, and weight considerations are relevant in the broader context of aircraft maintenance and design, the primary purpose of sealing fasteners and joints is to ensure they are flush and smooth for optimal aerodynamic performance.