I used a stub of a broken off Q-Tip to put a little sealant in every dimple. No leaks. Was it necessary? Probably not. Did it help? Maybe.
You'll have sealant between the ribs and skin, so nothing can get thru there. When you are done riveting, you'll goop over the shop heads, so nothing should get thru there. So except for air bubbles or missed spots, there should be no pathway for the gas to escape. But sealing over the shop heads and getting a good job is not as easy as it seems like it should be. I checked mine with a flashlight and a mirror, one rivet at a time and found I had to touch up several spots.
But here is how we did my tanks. I had 2 helpers. One with RV experience manned the bucking bar, I ran the rivet gun and an inexperienced helper gooped the holes and stuck the rivets in. So the pattern was, gooper gooped the hole & stuck in the rivet, I hit it with the gun, bucker checked the rivet, I pulled the next cleco & gooper did his thing again. This was with the tank standing on end. The first tank took several hours to do the ribs, the second one took about 2-3. Regardless of whether we gooped the holes, it was really nice to have 3 people.
Richard Scott
RV-9A Wings