What is a critical task performed during drydocking that involves removing old paint and rust from the ship's hull before applying new coatings?

The critical task performed during drydocking that involves removing old paint and rust from the ship's hull before applying new coatings is surface preparation, primarily achieved through abrasive blasting (also commonly called grit blasting or sandblasting). Surface preparation is the process of cleaning and treating the ship's hull to create an optimal substrate for the subsequent application of protective coatings, ensuring strong adhesion and long-term performance of the new paint system. Abrasive blasting is a mechanical method where abrasive particles, such as steel grit, garnet, or copper slag, are propelled at high velocity onto the hull surface. This process effectively removes all forms of contamination, including old paint layers, rust, mill scale (a layer of iron oxides formed during steel manufacturing), and marine biofouling (algae, barnacles). Beyond cleaning, abrasive blasting also roughens the surface, creating a microscopic profile or 'anchor pattern.' This profile is crucial because it provides mechanical interlocking points for the new paint, allowing it to adhere much more strongly than it would to a smooth surface. Without this proper surface roughness, new coatings would quickly delaminate or peel. Achieving specific surface cleanliness standards, like Sa 2.5 (a near-white metal blast) as defined by ISO 8501-1, is essential. This standard indicates that the surface is almost entirely free of contaminants and has a uniform metallic sheen. This level of cleanliness and roughness is paramount because any residual rust, salt, or loose material beneath the new paint will lead to premature coating failure, such as blistering, cracking, or further corrosion, significantly reducing the ship's protection and operational lifespan.