Why not do the balloon test first with the tanks on the spar? If that fails, remove and leak check. I built SB wings, so perhaps I'm missing something obvious...
The balloon acts as a pressure relief valve, and is not the leak test in and of itself as there's a number of reasons outside of a leak that could cause the balloon to deflate over time.
One of my QB tanks leaked. I filled them with water, and rigged a gallon paint can suspended 10 feet above to provide hydro- static pressure and let them set for a few days. First leak I was able to fix from the fuel sender hole. Rinse and repeat. Second leak took several days to show and required removing the tank from the wing and cutting a hole in the back to apply sealant along the entire inboard rib to lower skin joint. There was no sealant at that joint from the factory! How that got thru quality control I'll never know. Other tank was fine. Must have been sealed by a different worker.
Whoa - 10' of water head is a lot, around 5 psi! I suspect joints that wouldn't ever leak in service might eventually leak at that pressure. I'd limit is to perhaps 1' - 2' of head. Best is to use air, limited by a very fine pressure regulator (not the normal compressor types...), and a blow off column of water about 1-2' high, and use soapy water to look for bubbles.
Sorry for the crummy picture, but here you can see I've got a very low pressure regulator, and the outlet goes to a loop of tubing. I've put water in the tube, and pressurize until I get about 12-18" differential. Just to the right of the picture the tube is open, so if the tank were inadvertently over pressurized, it would just blow the water and vent air. There is about 2" differential when this pic was taken.
Cannot agree more with this statement. Simply attaching a balloon to an opening on the fuel tank and applying some arbitrary amount of air pressure in the tank is not a leak test. That balloon is just a pressure relief valve! The leak test involves thorough visual examination of all the seams, rivets, filets where air, H2O, fuel, or whatever, may be escaping. How you do that visual examination is the actual LEAK TEST! As you have seen here, and perhaps read elsewhere, there are many different methods of performning that visual examination. Chose one you are comfortable with and use it. But use it on the tank in a manner that will allow you to examine every minute inch of the tank. Doing a test with the tank attached to the wing is not going to cut it. You would never be able to inspect the rear baffle area (which incidentally, is where the vast majority of leaks occur).The balloon acts as a pressure relief valve, and is not the leak test in and of itself as there's a number of reasons outside of a leak that could cause the balloon to deflate over time.
I wanted to comment about the relation of tubing diameter to static pressure. It is the height of the water column that sets the pressure, no matter what the diameter of tubing. 10' of 1/8, or 1/4, or 1" tubing will all generate 10' of static head pressure.....
If you choose to use avgas to test, check your insurance first! My builders policy through falcon and global, is only valid until you put gas in the tanks...or so they tell me. Not to mention the fumes in the shop every time you open the tanks...