VertVega wrote:Hi and thank you all for the quick replies. I will try to find a convenient place to drill a hole and use a rubber plug after the treatment.
I have been busy searching that web site where I have seen the rusty "horror" photos. Below photo comes with the credits to "marsalek/xantia/prah " from where I have copied it.
It is a page from Czech Republic and I have no idea what is written there but just check the photos behind this link. Amazing, unbelievable and horrible.
http://www.tenzor.cz/marsalek/xantia/prah/
This must absolutely be a design bug

Hi VertVega,
and thanks for visiting my site!
There is also a continuation of the repair...
Both images Copyright user "jk" from citroeny.cz/forum
The trouble is, that the middle sheet gets firstly affected by the rust and then it propagates to the outside.
From behind, through the big rubber/plastic plug (see image below)

you do not reach the affected cavity!!!! You end up BELLOW the affected sheet!
One possibility to prevent the middle sheet from rusting or to slow down the process, is to use some penetrative product (we use Fluid Film ASR here with great results, it is based on lanoline). There is an easy way, how to reach the cavity.
From inside the car, remove the bottom bolt securing the seat belt and use a 6 mm drill (with its tip greased to catch the swarfs) to drill through the hole for the bolt. You DRILL THROUGH TWO SHEETS! (not more!) and you end up right above the rusty sheet. In the picture marked red. Take care, there is not much space behind the second sheed, say 5 mm. Otherwise your drill comes through the outside sheet.
After using FluidFilm ASR on my car (having no visible signs of rust), within two weeks the FluidFilm was comming outside between the sheets. I applied it 1.5 years ago and have still no signs of rust. The car is now 8.5 years old.
If I were you, I'd firstly thoroughly repair the paint on the visible parts and then immediately apply the Fluid Film or the likes. Only if this did not not help, next year you will have to cut the affected parts out and replace it by some home-made sheet, as there is no Citroen genuine repair set for this issue.
Karel Marsalek