I don't know if this is a problem in other countries, but in Norway the winter causes the government to order salting of the roads. The salt is known to cause rust, but so far I have heard about no MINIs with a rust-problem on the back door. This was until I checked my very own back door... :(

I saw some brown/orange substance over the "bar" which houses the numberplate lights and the back door-handle. I therefore removed the 4 screws which hold this chrome-"bar" in place, and found this:



The rust was worse than I had expected. Not only surface, but also piercing rust. I therefore removed the cover inside the backdoor as well, and i found more rust there, due to the piercing:



The worst of the rust was underneath the mentioned chrome-"bar". I am guessing this is because the seal isn't tight enough, and it might be preferable to fill the edges with silicone or something.
The good thing is that the rust is placed underneath this item, so you won't have to use too much time on finishing touches when fixing it. No one will see it anyway. The bad thing about this is that it is hard to notice.



Anyway,
what i did to get rid of this is that i used a steelbrush gently and some grinding(/sand)-paper. Of course you have to use maskingtape and such so you won't hurt other areas of the backdoor. I eventually hit bare metal in all the areas where there was rust, and i cleaned these surfaces before I sprayed with a primer that is rust preventative and rust eating. I sprayed several coats of this, and I of course let it dry a bit between the layers. After it was all dry I did some layers with clear-coat, just for extra protection. I was "lucky" to find a primer that wasn't too far from my pure silver color of the car. Had it been black is it possible that it would have been visible from some angles.


Big picture

BEWARE: the rust was not only placed under chrome-bar on the rear door. There was also rust underneath my numberplate. I urge you to carefully inspect the rear door of your MINI for this reason.

_____

If you use your car daily and don't have time for the primer etc. to dry completly, you can do like this:



The door is covered so dirt and rain won't hit the fresh paint, but be sure to put something in between so that the door won't shut completely. I think you would have a problem opening it again if you shut it without the opening mechanism attached.
On my car I put something between and strapped the door tight so that it couldn't open or shut completly.

Beat the rust.

 

I have seen quite a few R50's and R53's with this same problem. Most of the rust found behind the license plate is due to the license plate frame rubbing up against the paint and eventually allowing bare metal to become exposed to the elements. I recommend taking the license plate and frame off every few washes to inspect this area for wear and place some double sided tape on the trouble areas to prevent wear. Sand/Salt and other debris can also become lodged behind the plate and since it's not visibl, it gets forgotten, remember to clean this every now and again. I've also seen the rush you have pictured around the license plate light area. In the cases I've seen, it was much worse than your car. It had spread to a few areas around the trim and bubbled under the paint.

 

Thank you. In that case, I am glad i spotted it early. Any other places to beware of rust, besides the bottom drivers- and passenger door seal and the rear door?

 

To the best of my knowledge, the main issues are the boot lid (rear hatch), the area under the driver and passenger door seals. And on the early cars under the hood where paint can be rubbed off over time because of the rubber hood stops/bumpers.

 

I have rust above and behind the trunk handle like that. I'm planning on simply replacing the whole hatch, since I have at least 3 dents and some scuffed paint back there. It would not be cost effective to repair all the dents, rust, and repaint. I kind of want to get a carbon fiber trunk but I'm concerned about grounding issues with the radio and defroster screen. The way I see it, another metal trunk will eventually rust like this again. It is poor design in the handle. That foam pad stuff holds water like a sponge and keeps it all wet and in contact with the paint in the area. A bead of silicone might work though.

 

If you add a piece of metal to the inside of the carbon door you'll be able to ground from there. Or else you could just pull a cable through the excisting cablesock on the top of the door, grounding in the compartment on the right in the boot.

I agree, the handle is a bad design indeed. From what I gather, the french made it. But this may be wrong.