I completed the outer half of the bottom of the wing last night and found this when I was ready to move on to the next section...
What with the accompanying wind-swept blue streaks, RV veterans will recognize that immediately -- as I did -- as a fuel tank leak around a rivet. It's not uncommon, but it's disappointing nonetheless, especially since I used a lot of ProSeal (the sealer) on that tank -- almost an entire can, or twice as much as what most people use.
The rivet is along a rib line which just happens to be below the fuel cap opening in the tank, which at least allows me to peer inside and see if anything looks out of whack.
Behold...
That brown spot on the top of the stiffner certainly leads me to believe that something is going on there, but I'm not yet sure what. It's not where the weeping rivet is. The weeping rivet is on a rib line. Also equally perplexing is the color around the rivet head -- brown. Avgas is blue.
So what's next? First, of course, I need to drain the tank and I'm going to try to fix this without taking the tank off the wing. Then I'll poke around at that brown crud to try to figure out what it is and then I'll poke at that rivet head that's actually weeping to see if the ProSeal has broken down there.
I'd like to think throwing some more ProSeal on there (after attempting to cut out, perhaps with an Exacto knife duct-taped to a dowel) would solve the problem. But it's possible the route the fuel is taking is actually coming from the other side of the rib, which is not accessible.
There is the possible fix of using LocTite around the manufactured head and hope it "wicks" up into the area and block fuel, but I would think this is somewhat problematic given gravity and all. I could apply a slight vacuum, but this is a dangerous task because fuel vapors could ignite at the vacuum cleaner I'd use. Not good.
Alternately, I could remove the tank, cut a large access hole in the back baffle of the fuel tank, get in there and slather all sorts of stuff, and then put a removal plate over that hole and seal it tight. Of course, that introduces new possible points of failure.
I could also just try drilling out that rivet and using a long bucking bar to buck a replacement.
We'll see.
Hey, at least it's gotten me out of polishing for a few days.
There are safe ways to do the vacuum. For instance, assuming the tank is in thermal contact with the wing, you could put some hot, wet towels on the wing over the tank or warm it up in some other way. (In summer, you wouldn't have to do this.) Then seal the cap (including the vent) and apply bags of ice to the wing. A partial vacuum would be created. It's probably a good idea to provide some kind of relief to keep the wing from being crushed. Something like a really deep sink trap ought to work. You could also partially evacuate a 55 gallon drum and hook that up to the fuel vent, monitoring the vacuum, of course. A full vacuum will crush a 55 gallon drum. Or, if you can put the airplane at the top of a hill (doesn't have to be very tall), you could fill a hose with water, put one capped end at the bottom of the hill and connect the other end to the fuel tank vent. When you remove the cap, you'll get a partial vacuum. As I recall, a couple of feet of water is equal to about 1 psi.
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