Water tight. Not air tight, but water tight.

 I bet y'all thought I had quit and walked away.   Nope.   Thought about it a bit, but nope.

Both tanks remain stubbornly not-air-tight.  I was extremely hopeful that the through-bolt locking mechanism in the cap was the culprit.  I tried sealing the caps with rubber gloves, but there aren't any more latex rubber gloves and nitrile gloves don't have that stretch that is needed to make a perfectly smooth sealing surface.  They tend to fold and bunch up and that makes them leak way more than the cap by itself.  Condoms, on the other hand, are perfectly round in cross section and can be made to fit over the cap.  It typically took 3 or 4 condoms getting torn to lock the caps in place without damaging the condom itself, but I got good at it.

Still, the damn tanks wouldn't hold pressure.  I gave up on the balloons as pressure gauges and got some clear plastic tubing and food coloring to use as a simple manometer.   15 inH2O is about 0.5 psi and so I used that difference in water height as my pressure gauge.  Pumping up the tanks to 15 inH2O resulted in a drift down to about 1 or 2 inH2O in half an hour.  This was fairly repeatable.    Despite being repeatable, I could not find where the air was leaking using the soapy bubble water trick.  (Which is messy, by the way.  At some point, I'm going to have to haul the wing assembly out to the drive way and hose it off.)

I posted my plight on the forum and, man, did I ignite a firestorm!  Two suggestions were worth merit: one was that I had not accounted for the heat of compression and that the declining pressure was actually the heated air cooling off.  The second was to use dish soap rather than kid's bubble soap for my leak detection.   Both of these have been filed for potential future use.

Several commentators had used the logic of "it's a fuel tank, not an air tank" and simply filed their tanks with 100LL fuel and looked for the tell-tale blue stain.  Other's commented that that takes too long to see the stain, and others said balderdash and basically it turned into a debate similar to the holy war of priming vs not priming.

I like the pragmatic approach exemplified by the "it's a fuel tank."   I did a full up water test over the previous weekend.  Literally.   Lada & I filled both tanks to the brim with water and then left for a 3 day weekend.  Upon my return, there was no damp or wet spot on any of the sides or the tops.  I couldn't check the bottoms because of the weight of the water itself, but if either tank develops a leak on the bottom, I can repair those even after the wings are mounted on the plane.

Last night I drained the water and fetched 29 gal of 100LL from the airport.  I didn't have the correct hoses to configure everything (I can't use the cheap PVC tubing with 100LL, so I have to go to the hardware store for the proper fuel hose.).  I'm going to fill each tank and watch for 24 hours.  If no leak, I will declare them airworthy and get back to actual assembly.   This final step will probably take another week or so.  Stay tuned.