Which of the following is a function of gel-coat?

The function of gel-coat is primarily related to providing a smooth, protective outer layer for composite materials, which is essential for sealing and ensuring water tightness. Gel-coat is often the first layer applied to molds and serves to create a barrier that helps prevent moisture absorption and protects the underlying composite material from environmental factors such as water, UV radiation, and chemical exposure. This water tightness is crucial in applications like marine vessels where any water intrusion can lead to structural damage or degradation of the composite.

While other functions like strengthening the composite bond, providing insulation, or serving as a primer are important in different contexts, they do not accurately describe the primary role of gel-coat compared to its effectiveness in ensuring water tightness. Therefore, water tightness stands out as the primary function of gel-coat in composite applications.