Welding rusty metal with the wrong welding rod can have several negative consequences, including: Porosity: If the welding rod is not specifically designed for welding rusty metal, it can create small holes or pores in the weld known as porosity, which weakens the strength of the weld. Lack of penetration: The wrong welding rod may not be able to properly penetrate through the rust layer and create a strong bond between the base metal and the filler metal. Inadequate cleaning: If the rust is not properly cleaned before welding, the welding rod may not be able to properly penetrate through the rust layer and create a strong bond between the base metal and the filler metal. Inadequate slag removal: The wrong welding rod may not produce the right type of slag, making it harder to remove, which can lead to weak spots in the weld. Inadequate strength: The wrong welding rod may not produce a strong weld, which can lead to the failure of the welded structure. Poor corrosion resistance: Welds made with the wrong welding rod may be less corrosion-resistant and not last as long as they should. Overall, welding rusty metal with the wrong welding rod can result in a weaker, less durable and less corrosion-resistant weld, and can potentially lead to failure of the structure.