If you have a mk6 Fiesta and they are quite common still this may be of interest...
As usual on a nice sunny day I'm under a car dealing with rust
I have a 2004 mk6 Fiesta with low miles on which I'm doing a full suspension off clean up of the whole underside and all was well till I can across an area on both sides that seems to be a potential killer for the chassis rails due to nothing more than a sloppy bit of design.
The area is just behind the rear beam mounts (which I've removed) and is where they have put an extra sheet of metal over the chassis rail as some sort of pathetic bit of strengthening. I say pathetic because they have only attached it with about 4 very spread out spot welds and the sheet is only about 1mm thick. Cutting off the rusty metal revealed large slabs of rust due to crap being trapped between the metal, plus it slopes down and there is a hole in the rail to make sure water & salt get in! It's just a really terrible design.
Once all cleaned up it seemed the thin sheet had taken the brunt of any corrosion and it came up well so had caught it just in time. I've opted to not replace the cut out sheet as it was doing very little if anything. Both sides have now been done and I'm very glad I went to the bother of checking it out
Below is some photos to give you an idea - 
[Edited on 11/4/20 by Mr Whippy]