Introduction to Fuel Tank Sealant

 Been working on the fuel tanks all week but haven't accomplished a whole lot. The tanks seem to take extra time, and I'm not sure why. Originally I figured that without having to prime any of the tank interior parts the prep would go rather quickly, but that's not the way it has worked out so far.

 I started out last Friday by breaking a tool which slowed me down some as I waited for a replacement to be shipped. The plans have you countersink (my favorite build activity) the tanks rear baffle. This baffle is the last item to go on, and as such you have no access to form a fillet with the fuel tank sealant. The process used is to layup a pretty healthy bead of sealant on the skin, and then drop the baffle straight down. This should push the bead of sealant ahead and create a good seal. Obviously having to spread the skins to allow skin dimples to pass and then seat could cause a problem, so we countersink. Anyway, I had finished one side of the skins and decided that I could use a break for a few minutes. I knew that I would need to countersink the fuel cap flanges for skin dimples so I decided to knock these out. While countersinking the final hole my hand steadying the flange slipped allowing the flange to tilt and snapping the pilot off of my countersink cutter. I ordered up two this time so that a future breakage wouldn't slow me down.

 

Fuel tank cap and flange. The countersink cutter broke while cutting the incomplete countersink on the right.

 I wasn't comfortable placing any of the ribs without all of the countersinks done on the skins, so there was no rush getting everything preped. Over the next several days I dimpled the left tank skin, got all of the contact surfaces for the left tank roughened and then cleaned well, and continued on parts prep for the right tank.

This past Friday we got our first taste of working with the tank sealant. With the short stiffeners ready to go, and relatively easy to do, we decided to go for it. The first step was to clean the contact surfaces very well so that the sealant would be able to make a good bond. The plans called for using MEK but it seems that it's hard to get in California, so I used acetone instead. After the acetone had evaporated I masked the areas where I didn't want to get sealant, leaving enough space for a good fillet seal around each stiffener. I didn't mask the area where the ribs will go later, thinking that I would just clean those spots afterwards. I'm not really happy with the way that turned out, and I will be masking the rib spots on the right tank for sure.

Left tank skin with the stiffeners riveted and sealed and the masking tape still in place

With everything clean and masked we mixed up a batch of sealant. The sealant is a two part system and claims to have a working time of about 2 hours. I had heard that this stuff smells bad, and they weren't kidding. When I opened up the can of base it smelled like raw sewage, in fact if it weren't for the white color and thick consistency you could easily think you had a can of sewage. After mixing in the accelerator either the smell got a little better or I was used to it. I wasn't sure how much would be needed for each stiffener so we mixed up about 35 grams in the first batch. In the end we would need another 35 gram batch to finish up.

Left tank skin with the masking tape removed

 At each stiffener location we ran about a 1/16" bead all the way around the rivet holes and then a small blob between the interior holes. We then laid the stiffener and placed a cleco in every rivet hole. The clecos did a good job pulling the stiffener in tight and sealant squeezed out of all sides. I then used my finger to form a fillet all the way around each stiffener. There were only a couple of places where we needed to add a little sealant to form a solid fillet.

Once all of the stiffeners were clecoed on we moved the skin into the cradle and began to rivet. The process was to pull a cleco, place a small dab of sealant in the hole, push the rivet thru, and then drive the rivet. This should have created a good seal around the rivet dimple and hole, and after driving there was a small sealant ring around each rivet head.

Rivet shop heads sealed

After all the rivets were driven we used the remaining sealant to seal the shop heads of each rivet. This will hopefully form a complete seal of each stiffener. I then cleaned the excess sealant from each location where ribs will pass thru, and then pulled the masking tape. I feel like the fillet seal may have been compromised while cleaning for the ribs so I'll be sure to place extra sealant during rib placement. When we get to the right tank I'm going to be sure to mask the rib locations as well.

I also received the new countersink cutter Friday, so I was able to complete all of the required countersinking today. Tomorrow we'll start with the left tank ribs. All in all I don't think our first run with the sealant was too bad, certainly not the disaster others have made it out to be. The sealant certainly smells bad and is a sticky mess, but if you take your time and watch what your doing it's alright.