Thanks for the answers sofar. I'm from Europe (Netherlands) and for the technical matters I'm not sure if we have to do with the European regulations (JAR / JAA) or the Dutch national regulations. The aircraft original was equiped with the normal standard integrated tanks and was later modified with 2 bladder longe range tanks. No it's not possible to seal the rubber bladder tanks. Already 2 times our maintenance firm ordered new bladder tanks because of leaks / dry rubber with little cracks. It's really a pain for us and we would be very happy if those tanks (wich we don't need at all!) are removed.
What is AFM? Has someone technical experience with removing bladdertanks on a C206? The reason why I post this topic here is that we have a little discussion about his point with our maintenance firm.