Hi,
Thank you all for this thread and for the valuable information.
I would like to share my experience with this issue, so it can inspire others and possibly help them.
I have worked on a friend's R1200R 2012.
It all started when we saw the heads of the screws of the fuel tank filler neck all rusted, when he was filling his gas tank. I told him we need to change the screws, but that there may be underlying problems. We then found this thread. Sure enough, when disassembling the filler cap, there was rust everywhere and it was starting to flake so it really was time to do something !
First, I used a dremel tool to remove as much rust as possible.
Safety advice : hermetically plug the gas opening (see below) when you do so, because of possible sparks, either from the motor of the dremel tool or from the tool tip itself.
Then, I used "Rust Check" liquid bottle rust converter to try to convert as much rust as possible. I'm not totally excited by this product, it seemed to be pretty weak, but we managed to convert some of the rust which is better than nothing, after several coats and letting it dry.
I then used an automotive paint degreaser (based on acetone) to remove all chemical residues and prep the surface.
Then I applied a fair amount of Rust Check silver grey rust paint. It's supposed to be OK on top of slightly rusted metal. I put 3 coats, a first light one, then 2 heavier ones. I put quite a bit of paint because the surface wasn't totally flat due to the rust flaking a bit, and I needed to make sure everything was covered (didn't have POR15, looks thicker so it would have been better I guess).
And then it took AGES to dry. 24h after, it was still not dry !!!
My friend was not very happy, he wanted to ride his bike, which I can understand.
I believe it was because of the fuel vapors dissolving the paint and preventing it from drying.
I took inspiration from this post :
Not happy!! My next decision was weather to drain and remove the tank or fabricate a plug so no nasty bits fall in the tank
http://i1241.photobucket.com/albums/gg504/ejute/IMG_3746_zpsb58f7885.jpg
I made a similar setup (using old circuit boards, I didn't have a suitable piece of metal available) :
(see attached picture)
At this very moment, it's still drying and I will make another post to report the end result, tomorrow.
Of course we'll add the "abdeckring", which is currently on order at our local BMW dealership. But it's nice to have several layers of protection.
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Now, regarding the screws.
Initially, we wanted to replace the screws to stainless steel screws, but before doing so, I tried fixing them. I put them into hot vinegar for some time, which removed most of the rust. Then I painted them black with krylon matte black paint. They look great now.
(see attached picture)
Normally you don't do this because the rust changed the shape of the threads and there will be uneven torque among the screw which can lead to problems, but there is not a lot of force on these fasteners so I believe it was OK to do it here. Similarly, hydrogen embrittlement could be an issue if the fasteners were subject to a lot of force but it's not the case here.
The screws will be inserted with Loctite marine grade anti-seize (hard to find but best stuff) to prevent any further rust on the threads and prevent possible galling (would have been an issue especially if we went forward and moved to stainless steel fasteners I believe).
I also fixed this way the 4 smaller screws at the top of the cap. Looks much better.
(see attached picture)
That's it for now !
PS : Here is the stuff I used so far (the rust converter is in a glass jar, this bottle is crap and leaks, be careful) :
(see attached picture)
