I have the problem too, and researched this a long time ago. I slow built my tanks and all rivet shop heads are sealed inside. I have no fuel leaks at all (740+ hrs). I had the plane professionally painted by an aircraft painter. The paint blisters appeared only on the top of my tanks on the rivet heads and took several months after the painting was complete to manifest itself. After much research, I finally concluded it was a reaction of the paint thinner/solvents used in the primer/paint that reacted with the proseal causing trapped solvent gases to eventually cause the blistering. It took the heat of the summer as well to create this condition. Google "trapped solvent", "paint thinner blisters", "solvent popping" or some such. Apparently, some painters may use paints and thinners that aren't compatible or aren't compatible with proseal. Even the thinner in the paint, not the solvent used to clean before painting, can be the culprit. Bear in mind that many production planes don't have this issue because they use fuel bladders or internal tanks, not a wet wing sealed tank like we have. Here is a link to one of the resources, look for "Solvent Popping".
http://www.tat-co.com/tat-co/documents/paintdefectguide.pdf