Clean the surfaces well - since it contains threaded areas, you might need a brush since you wouldn't reach all areas with an emery cloth. Flux, then solder. If you don't get it clean, there are no guarantees the solder will flow. If this is going to be an electrical rather than a plumbing connection, you could use an electrical solder which can be bought with a non-acidic flux. That would have lead in it (probably, but there are lead-free electrical solders, too), and may flow more easily. You still need to clean things well first.