Bits and Pieces
Homebuilders Tip - Tank Sealing
By Ian Brown, Editor - Bits and Pieces, EAA #657159
Any builder who has built an aircraft with "wet wing" tanks will know the trepidation with which the test is begun to find out whether the tanks actually hold without leaking. You definitely do not want to test them by filling them with avgas in case a leak needs to be fixed or filling them with high-pressure air which can cause seams to blow out. Testing with low-pressure air, in my experience, is fraught with the risk that you don't actually know whether a slow loss of air is due to a leaking balloon or a bad seal around the filler cap or whether it's the tank itself.
In the end it's very easy to wind up with a leaking tank and potentially quite a horror to fix.
This is the result of not finding a leak in time.
To avoid removing rivets or disassembling anything, first find out where the leaks are coming from. Pressurize the tank with a balloon filled with air, on the fuel entry point, blocking the vent and filler cap. Use soapy water and look carefully. You might spot a fine trail of bubbles leading to the tell-tale spot that didn't get enough Pro-seal.
Once you've identified where the air is leaking you can attempt to reseal by pouring diluted Pro-seal through the filler cap and allowing it to flow along the seam in question. Methyl ethyl ketone (MEK) is one recommended solvent, but it's not freely available in Canada. Lacquer thinner seems to work well (it does contain MEK), and with the right quantity, it will make Pro-seal nice and runny but will set up very well. Do an overnight test to convince yourself that you have the quantities right. Mix a small quantity, then put a drop on a flat sheet of aluminum. You will be able to see how it flows and how it sets up.
Once you're ready, simply pour the runny Pro-seal through the filler cap. Orient the tank so the liquid will flow along the problem seam, then set it so that, as the solvent evaporates, you will have an even amount spread along the seam in question. You might read many more solutions to this, including cutting large holes in the baffles, but try this tip first. You might be very pleased you did.
One last thing. Make double sure you have leak-free tanks before you mount them. On my RV-9A they are a bear to remove!