Fairing
Fairing is the work of making the surfaces of a hull smooth and continuous, so the shell forms a clean, flowing curve. In design, lines fairing refines the hull surface until every waterline, buttock and section is fair; in the yard, structural fairing corrects local unevenness and weld distortion so the finished shell is smooth.
A fair hull matters for both hydrodynamics and construction: an unfair surface increases resistance and fuel consumption, and a badly distorted structure is hard to assemble and to coat. Modern surface-modelling and controlled welding reduce the amount of physical fairing needed.
Fairness is quietly worth money for the life of the ship: an unfair, wavy hull carries extra drag and burns more fuel every voyage, while distortion left in the structure slows assembly and shortens coating life. Getting the surface fair is both a performance and a build-quality issue.
What is the difference between fairing and hull coating?
Fairing makes the underlying surface smooth and true; hull coating is the paint system applied over it. A fair surface lets the coating perform and keeps hydrodynamic resistance low.