I don't have the tank yet, but I have to believe the design would take the venting above the fuel level like the main tank so fuel is not introduced into the vent line. The check valve I read about might have some function here. Sorry I can't be more specific, too early to know. A call to Touratech might answer your question. Every time I have called I have been given no BS answers.
As for the porosity, this is a Polyamide (PA6) rotation molding material that is rated to seal a number of chemicals including gasoline. The manufacturer, Elkamat, is the supplier of fuel tanks for a number of vehicle manufacturers including Porsche, BMW, KTM, and Yamaha. These manufacturers must ensure their tanks meet EPA regulations concerning vapor containment. They make fuel tanks for production runs as well as speciality runs. They have supplied Paris Dakar teams from KTM and BMW. I think the tanks will be solid and non-porous. On their website they note Touratech as one of their customers. This is a very different material than used for the under seat tank of the F800GS. It a material also used for high strength car bumper structures.
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