I am currently adressing the blister situation with my left tank. The right one is blister free. Both were built by me and not at the same time. I am convinced that I was not careful enough with sealant regarding shop head encapsulation and wet installing rivets.
So....since I am fixing up other issues with the plane, I decided to sand down the paint over the tank rivets and smear some proseal over each head ensuring that it gets between the rivet and the side of the dimples. Some of my rivet heads had gaps between the edge and dimple.
I know that sealant repairs should be done from the inside, but I am taking the simplest route first.
I will prime over them soon and report the results.