Wow, that's more than a few bubbles! and this is on pressure from a balloon? Can you locate the source as the bolt heads or seal around the sender plate or multiples locations? It's hard to tell from the photo.
Your test photo shows a couple of concerning places: The bolt heads are not bedded in sealant, the seam between the end bulkhead and tank skin doesn't show any sealant squeeze out, the edge of the sender plate has gaps without squeeze out.
I looked back at some of your previous photos to see if there are any obvious glaring issues. Question: what type of fastener is on the fuel side of the sender plate bolts? They look to be standing proud of the pro seal and not capped off by the sealant. Maybe it's a new kind of fastener, but that arrangement doesn't look familiar.
Any way you slice it, you'll need to do some serious sleuthing and figure out the source of that leak. Hopefully it becomes clear soon.
Bill (Wild) VA-165 '90-'93
First Flight 7/17/2021!!
- Been there, Donated 2022