The fundamental concept behind a fairing compound is that it is a material added to a surface that is not level or smooth, in order to fill the low spots, then sanded smooth to blend in with the surrounding surface. Surface fairing can include fine surface leveling, filling of screw holes, small dents, divots, or other small indents. These materials can come pre-made, optimized for certain substrates or applications, or they can be customized.
Fairing materials are comprised of a resin system that acts as a glue or adhesive, and fillers or thickeners to give it body, sag resistance, and sanding qualities. The resin material bonds the filler materials, and gives the product its bond strength to the substrate. The ideal fairing compound has a good balance of ease of spreading and applying, ease of sanding once it is fully cured, a great bond strength to the substrate, and good dimensional stability. Usually, fairing compounds will have a consistency like peanut butter, and will not sag on a vertical surface.
Epoxy-based fairing compounds promote the best bond strength, and greatest dimensional stability, while polyester fairing compounds can be ready to sand very quickly, and are generally the lowest cost.
In most cases, a primer will be applied over fairing compounds, prior to being overcoated with a finish material.