I don't want to derail or hijack the thread, so let me start by saying this. Building an RV-12 fuel tank does look like it would rank among the toughest jobs. I built two RV-7 tanks, which are "fun". Since the 12 uses blind rivets you won't have quite as much mess and hassle. Working with sealant can be messy, but I've seen guys who can keep it under control and do a very neat job. Just remember that ANYTHING that isn't properly prepared, cleaned and sealed can and will leak.
In this case the leak turned out to be worse than we initially thought. During the prebuy the tank was not full and I didn't pull the baggage area carpet out (lesson learned). When we picked it up the tank had just been topped off and it smelled pretty badly. We found a very minor seep around the sender and tightened a couple of screws, and figured that would fix it. It just got worse on the way home. It was only when I pulled everything out of the cabin that I found the extent of the problem. It's obviously been leaking for a while, but only when the tank is fairly full. I won't know the exact location of the leak until I get the tank drained and removed, but I suspect it's near the top of the sight gauge (which is going away).
I don't know that I would consider the plane not airworthy, but I'm not flying it again until the leak is fixed. It's what I would consider severe. I'm still trying to air out my flight bag and the headsets. If there's a good way to get mogas smell out of stuff, I'm all ears. The baggage area will get stripped and painted with epoxy primer. Most of the carpeting is OK after airing out and a good scrub with Woolite foaming cleaner, but I may just have to toss the baggage area carpeting. It's work, but I knew what I was buying and it was priced accordingly.