Thanks to everyone who is posting on this topic. The information is invaluable. I am getting ready to assemble my fuel tanks and am experimenting with different combinations of dimple dies and assembly techniques before I do it for real.
So far, the combination that has produce the best looking results is spring back dimple dies on the skin and tank dies on the substructure.
I also sliced some of the samples to see what is going on under the rivets and was surprised by what I found. Here's a typical picture:
The porous structure of the sealant is evident, and it is very easy to see how leaks are going to develop if the shop head of the rivet is not sealed properly. After looking at this, I'm not sure how much added leak protection is provided by the sealant that is under the rivet.
In fact, it looks like the added sealant under the rivet may just temporarily mask a faulty shop head / fillet job that will end up leaking after a few hours service.
Please share any observations, insight, and experience you have. I'm trying to do this right.