What must be done to existing base metal to ensure quality for new welds?

To ensure the quality of new welds, it is essential to thoroughly clean all foreign matter from the existing base metal. This cleaning process removes contaminants such as dirt, grease, oil, rust, or paint that could lead to defects in the weld. When contaminants are present, they can interfere with the fusion of the weld metal and the base metal, potentially resulting in weak welds, porosity, or inclusions.

Cleaning the base metal prepares the surface for optimal adhesion and helps to create a strong, sound weld. Various methods, such as grinding, sanding, or using solvents, can be employed to achieve the necessary level of cleanliness depending on the condition of the base metal and the type of welding process being used.

The other choices suggest incorrect approaches to ensuring weld quality. Applying a coat of primer could create barriers that limit weld penetration or lead to issues with the weld itself. Warming the base metal may be necessary in certain situations, like for specific alloys or in cold conditions, but it does not address the fundamental requirement for cleanliness. Additionally, suggesting that nothing needs to be done is not aligned with best practices, as surface preparation is a critical step in welding.