This session started by match-drilling the J-channel against the existing holes in the left fuel tank skin. The J-channel is used to add rigidity and avoid oil-canning, but it is supplied from the factory without any holes.
Then the rear baffle is clecoed into place in between the left fuel tank skin so that the skin can be countersunk for flush rivets. I found it interesting that Vans choose to countersink since the skin is dimpled elsewhere for the rest of the ribs, but there's probably some structural or leak-reduction reason for it.
After that, all of the screwholes along the trailing edge and sides were final-drilled to #19 and deburred. The J-channel was also removed, deburred, and dimpled. Next all of these steps were repeated with the right fuel tank.
What's next? I think the instructions call for dimpling of all of the hundreds of holes in the fuel tank skins, so that'll probably consume most of my next session.