Exam Question

The preparation of a ship's underwater hull before applying a long-life coating in a dry dock involves a three-step process. This process addresses the removal of contaminants and the creation of a suitable surface profile.
(i) Washing: The hull surface must be thoroughly cleaned to remove all marine growth (algae, slime, etc.), accumulated salts, dirt, grease, and oil. High-pressure freshwater washing is the standard method for this initial cleaning. The goal is to present a clean substrate for subsequent stages.
(ii) Blasting: Abrasive blasting is the preferred method for removing rust, defective paint, and any remaining contaminants. This process achieves a bare metal surface, essential for proper adhesion of the new coating. The extent of blasting (localized or full hull) depends on the condition of the existing surface. The intensity and type of abrasive used are carefully controlled to achieve the desired surface roughness profile.
(iii) Primer Application: After blasting, the surface is again cleaned to remove any blasting debris. A primer coat is then applied to provide corrosion protection and to create an ideal surface for the subsequent topcoat adhesion. This primer acts as an intermediary layer, enhancing the bond between the substrate and the long-life coating system.