In preparation of installation of the top skins, I had to put the fuel tank on the spar.   I remembered I still had to cover up some rivets heads on the tank baffle and replace some of the rivets.

I first installed the cork gaskets on the access doors and tried to pressure test the tank. I soon found out that the cork did not stop air from going out via the access plates.

To avoid any other problems with them, and after inspecting the interior of the tanks one last time, I decided to seal them with proseal without the cork gasket. The cork gasket is known to cause leaks after 4 to 5 years.

Here is the sealed fuel tank sender access plate on the tank baffle.

There is an inconvenience with this approach of not using a cork gasket. The flop tube has a shelf life and needs to be replaced after a number of years. In that case, I'll have some annoying task of removing the sealed access plate.

Then gave it another try.

Installed the fuel tank test kit which consists of a cover nut for the fuel line, a pump valve that goes in the drain point through which you can inflate the tank. I also made a smal pipe that is mounted with regular flare and nut on the vent line to install the balloon.

Also taped the fuel cap with duct tape.

Be carefull when pressurising the tank, especially if you use a water level. The pressure in the tank may quickly exceed acceptable levels causing permanent damage to the  tank assembly. Go easy with the air input and do not use a air gun. Just use a bike pump. That will do it. You will be surprised how little air you need to put in to inflate the balloon. 

The balloon was fine for the entire evening, but ... the next day it was deflated. damned !

I'll have to try this again with the tank off the spar and using soapy water to test for small leaks.