Every car has an economical service life, the Xantia is at the point of no return now. They are cheap bangers to most of us (including myself) that are good every day cars. Only the sickest of freaks would attempt a restoration on one of these.
In answer to your OP, you can prevent rust with waxoyl and schutz but if it is already there these will not cure the problem and unless you grind out / graft in new metal and rebuild the cars days are numbered
increasing Xantia life !
Moderator: RichardW