What is the primary purpose of the fill coat on a fabric-covered aircraft?

The primary purpose of the fill coat on a fabric-covered aircraft is to finish and seal the fabric. The fill coat is a critical layer that helps to create a smooth surface over the underlying fabric, filling in the weave and imperfections. This not only provides a uniform appearance but also protects the fabric from environmental factors, including moisture and UV exposure.

While some finishes may involve aluminum pigments to offer UV protection, the overarching role of the fill coat encompasses sealing the fabric to prevent deterioration and wear. Thus, it ensures the longevity of the aircraft covering and contributes to the overall safety and aesthetic quality of the aircraft's surface.