Having owned my first 5 for just six weeks and, with winter approaching, decided to use this half term holiday to thoroughly check and prep it. Firstly, this is a 16+ year old UK model and, even though it all appeared good, I was not at all surprised to find some corrosion. The corrosion would have been very difficult to spot given that it was well hidden under the factory underseal and behind the arch liners and I doubt that any seller would be too happy if asked to remove the arch liners so that you could remove the underseal to check for hidden corrosion. The corrosion seems to be partly caused by the poor design of the front and rear wheel arch liners (as well as being an old ‘biddy’ of course. These have been modified as shown below .
The Rear Liner
The rear liner is secured by three 10mm bolts and a self tapping crosshead. One of the 10mm bolts and the self tapper are in the wheel arch well and these proved to be ‘pigs’ to remove. Using a hot blade, I had to cut the liner around the securing points. Eventually they were out, the other two came out no with problems. On first inspection everything appeared OK but behind the liners was a lot of compacted dirt. and, with suspicions aroused, decided to use the angle grinder wire brush attachment to remove the factory underseal. The whole area beneath the underseal was rusty and corroded.
Picture 1 
The extent of the corrosion can be seen in the first picture (and it was the same for both sides). The yellow arrow points to the remnants of the hole where a large rubber bung is located. This makes an excellent point for injecting anti rust treatment. However the bung sits on a section of the panel that has a raised contour and the liner has a relief to enable it to sit over the bung. This raised section in the panel and the relief in the liner allow dirt and moisture to accumulate and create a mud trap. The white arrow shows the vertical seam and this is poorly protected by the liner as water will cascade down from the open section at the top where the liner starts. The Blue arrow shows where mud and water will collect and I think there should be a drain hole here. The liner is only tight to the arch around the two securing points and dirt and water gets trapped around them, hence the difficulty in removing them. At this point I had decided that the liner will have to be modified,
Picture Two 
I make my repair patterns using a paper template, then transfer the template to mild steel sheet and cut using a Jig Saw. A hole was punched for the grommet (albeit slightly smaller than the original) and the new section mig welded into place. The yellow arrows show a few of the many overlapping seams that exist around and under the rear of the 5, these will need to be scrutinized and treated as water will creep into them.
Picture Three 
The repairs are now completed and the rear underside cleaned, primered, sealed and painted, etc. The rear sill was liberally injected with rust inhibitors and the grommet replaced. I have always used and applied ordinary grease (lots of it!) around all seams, flanges grommets, nuts, etc and pack it in. Sometimes I add a top coat of underseal, or ‘waxoil’ ( sometimes I just use old engine oil and spray the complete underside ….you will never see a rusty Mini front subframe….its covered in oil!) Anyway, each to their own but the arrows do show where I have applied a generous amount of grease particularly along the flange marked by the blue arrow).
Picture Four and Five – The Rear Liner! 
I have removed the offending section, note the raised contour that sits directly over the original grommet which seems to trap dirt and moisture. The liner is now secured by the two remaining 10mm bolts and the section that covers the flange remains intact. Picture five shows the liner in place, yes the area is exposed but I prefer to be able to quickly jack the car up and easily inspect, clean and maintain it without having to remove the wheels or the liner. I shall, however, be fitting a new liner similar to the one fitted to the front as detailed below.
The Front Liner 
I have no pictures of the repair but once again the front liner seems inadequate. I have omitted the securing screws that pin the original liner on the vertical section of the front wheel arch (for the same reasons as for the rear liner), blanked the holes and added a second liner as can be seen. The liner is made from 1mm PVC and available in sheet form (and very cheap!). This a very simple mod that is easily carried out and I have used it on numerous restorations quite successfully. The liner extends from the mid point of the top of the front wheel arch and extends down and beyond the front sill rather like an extremely long mud flap. I have used the existing securing points along the wing flange and a few tie wraps to the existing liner. Next step is to repeat the same for the rear arch. The liner can easily be removed when the weather gets better and quite frankly, if they only survive a few months they are so cheap to make it won’t break the bank! The sills appear OK but I have no doubt that come the Spring they will be off and and a proper renovation will commence.