After a little success on the stiffeners I sealed and back-riveted the fuel cap flanges, then sealed and riveted drain valve flanges. The only tricky area is making sure the fuel cap flange and skin are pressed down tightly against the back-riveting plate when setting those flage rivets. Here’s the right tank fuel cap flange. No pictures of the left tank cap, but you get the idea.

And here’s one fuel flange sealed and riveted.