It is most important to properly prepare the surface to be welded because a satisfactory weld cannot be made over thick paint, rust or marine growth. The initiation of an arc may be impossible or, at best, very difficult when the surface has not been made ready for welding. Even a slightly rusty surface will adversely affect the quality of stainless steel and high nickel welds. Surfaces to be welded and adjacent surfaces for a distance of approximately 1/2-inch must be cleaned to sound metal.
Plates or patches should be prepared or cleaned aboard ship or above water with a sander, grinding wheel or by flame cleaning before being lowered for fitting and welding by the diver. Cleaning the base metal surfaces underwater necessitates strenuous application of the scraper, chipping hammer or wire brush. Mechanical methods of cleaning, such as a high-pressure water jet cleaning tool or pneumatic or hydraulic tools, should be considered to aid in cleaning underwater surfaces.
In a multiple pass weld, each bead must be thoroughly cleaned and inspected for defects before depositing the next bead.