I did some more work on my fuel tanks today and I sealed the end ribs to both tanks. I used a cheap 1" brush from Lowes, (that's a B&Q type shop for my UK readers), I cut it's bristles down to half length, so it could be used as a stippling brush. I covered the ribs' edges with the sealer and then installed them into the tank skins and clecoed, I'll rivet them tomorrow.
I decided to install the innermost ribs even though I do not have my pipe flaring tool yet, but as the aluminium tube used for the breather is so soft that it could still be installed after the inner rib was riveted in place, I decided to go ahead.