YIHAAA! Turns out the left wing tank is leak free,
I used the cardboard box of the empennage as floor and put the left tank on the baffle (tank attach angles). Than I filled the tank through the fuel caps (with water).
After one hour of preparing the wing skins (deblueing the rivet strips). I found no droplets on the cardboard.
After removing the water (through the drain), I inspected the baffle and tank attach angles and found no leaks.
Next time we’re going to close the right wing tank.
Time: 5 Hours, Rivets: 0/0