The way the load of the battery is transmitted through the bulkhead puts stress into parts of thin metal that can’t really handle it.
At some point in the past someone had applied some repair panels to the bulkhead round the lower mounting points. These were untreated steel that had been liberally pop-rivited and had inevitably picked up surface rust.
Given the number of rivets, removing the panels isn’t really practical so the next best option is to clean them up and paint them.
Usual techniques apply here: remove as much of the surface rust as practical, treat with rust remedy, mask up, etch prime, prime and top coat.
This is another case where Citroen’s legendary build quality shows up. I’ve painted the patch panels with the correct Blue Nuit paint used on the body but this section of the bulkhead got little more than a cursory blow over at the factory – certainly not enough to cover the undercoat!
Also visible here is one of the cracks that had formed on the lower edges of the battery box. I’ve drilled these out to stop them spreading any further and treated them with Waxoyl to keep the tin worm at bay.