The last time I bought sheet metal my choices were Galvanized and Paintlock. I am cutting small pieces and spot welding (electric resistance) them together. I chose the Paintlock after the salesman told me it would not interfere with spotwelding. He fibbed.... I've tried soaking in Acetone, sandblasting, wire-wheeling and an angle grinder with sanding disks. The angle grinder is the only one that worked worth a darn in removing this crap. I don't think it is a coating, but rather a surface treatment of some sort.
Does anyone know another way to make this stuff weldable?