How must the surface of new and just processed metal be prepared before welding?

To ensure a strong and reliable weld, the surface of new and just processed metal must be prepared thoroughly. This involves removing any mill scale, which is a flaky substance that develops on hot-rolled steel during manufacturing. Mill scale can hinder the weld's ability to penetrate the base metal and create a solid bond, leading to defects in the weld.

The correct approach emphasizes that the surface should not only be clean but also free of any contaminants that could affect the quality of the weld. Using vigorous wire brushing is essential as it effectively removes the mill scale, rust, and other surface impurities. This thorough cleaning allows for better adhesion between the weld metal and the base metal, leading to a stronger and more effective weld.

While it is important for surfaces to be free from oil, dust, or other contaminants, the presence of mill scale specifically makes option C the most comprehensive answer, reflecting the requirements outlined in the AWS D1.1 code for surface preparation before welding.