Time to mount the right tank to spar and to do an extra leak test.
Sling builder Alois from Germany gave me a bottle Ardrox AV30, a product that TAF is using for corrosion protection of the mating surfaces. Especially the top fuel tank skin has only a small overlap with the top skin of the wing. To prevent water entering here I used this product:
Todo: is replace the AN3 bolts under the ticker shims with longer bolts.
See Fuel tank leak test for more info.