I own a Seicento 2002. It's in good condition but there is rust on the underneath at the rear of the car. Some is in the spare wheel housing, which I am not too worried about as I expect it can be repaired or a new section welded in. But there is rust on the ends of the chassis box section (sorry if I've not used the correct name for this). It still has its structural integrity, but I want the car to last and so would like to know whether this can be repaired and how to do it. As I can't see inside the box section I can't treat the inside nor see whether it is rusted through. Is this a welding job, and is it even feasible to replace these structural sections?

I'd say just scrap/wire brush off the loose rust then brush on a coating of grease mixed with oil. Oil helps mixture to climb into crevices. Spray same into the box sections, with what, well make something up.

yes Oil helps mixture to climb into crevices.