Why must metal that has been used before be cleaned prior to welding?

Metal that has been used before must be cleaned prior to welding to remove contaminants such as rust, dirt, oil, and paint, which can negatively affect the quality of the weld. These impurities can lead to poor fusion, increased porosity, and weakened joints, compromising the overall integrity of the welded structure. Cleaning ensures better penetration of the welding material and promotes stronger, more reliable welds. It also helps in achieving consistent and uniform heat distribution during the welding process.