I haven't read this thread in its entirety so I'm not sure whether this has been suggested elsewhere already:
( Galvanic Corrosion:
When two different metals or alloys are immersed in a corrosive solution or regularly connected by moisture, each will develop a corrosion potential. If the conditions for galvanic corrosion are present, the more noble metal will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. If this occurs, the anode's rate of corrosion in the service environment will be increased while the cathode's corrosion rate will decrease. The increased corrosion of the anode is called "galvanic corrosion" )
My thinking was that aluminium aka the roof panel and ferrous metal aka the reinforcing strip in the weather seal can coexist happily without the aluminium being corroded but only in a moisture free environment. It is the presence of moisture that creates a situation in which galvanic corrosion occurs the roof panel aluminium being the sacrificial anode in this scenario.
We had our 2012 California repaired in October 2014 under warranty (front section replaced and lifting roof panel repainted). We then moved to France where in April of 2015 I noticed a few tiny bubbles reappearing under the weather strip where it had been repainted but only over an single 5cm section Knowing that dealing with VW Commercial in France was going to be another challenge altogether I decided to pack the weather strip with a good quality clear silicon grease that would be inert to paint and rubber but would keep any moisture out and reduce the chances of more bubbles appearing.
Well I am pleased to say that now after removing the weather seal and and residual grease the very early corrosion is no worse and is confined to the same 5cm area. I have repacked the weather seal with the remainder of the tube of grease and will check again in another 6 to 9 months. I am confident that this is a less than elegant solution to a problem which otherwise won't go away. BTW the front panel thankfully still looks OK.
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