In general the firewall sections didn't have too much complication. Applying tank sealant to all the seams was a bit of a pain, but not particularly difficult after building the fuel tanks. The hardest part was probably getting the bends for the stainless steel sections at the tunnel entrance to align and lap correctly. Once attached together, the parts are very stiff and difficult to manipulate, and it actually required quite a lot of pushing and bending to get everything to align right. Once it all snapped in place the fit was actually pretty good, but the clecos were somewhat insufficient to hold it all together. I had to create a few shims with #30 holes to keep the clecos from pulling out on the curved section of the tunnel while setting rivets. Otherwise, the only modification was to leave out the exhaust brackets since I elected to use the EXP119 engine, and instead I just put blank rivets in the holes that would normally hold the steel brackets. I kept the doublers behind them, however, figuring they still provide some rigidity and structure.

I've found that spending the time upfront to make sure that all flanges are at their proper angles, and that all fluting is performed such that hole patterns line up exactly without preload has saved a ton of time dealing with it after parts start getting assembled. Here, the outermost ribs follow a curve to follow the contour of the fuselage that needs to be carefully fluted to match the hole pattern in the bottom skin.

The idler bracket gave me some trouble, given that a 45 degree bend has to be made on the diagonal edges very close to the edge on relatively thick material. A hand seamer was simply unable to make this bend without either badly scarring the material, or resulting in a bed that is not at all sharp. Instead I fabricated a bracket out of angle aluminum as a backing plate, clamped the assembly down to a table, and used a combination of the hand seamer as well as hammering the edge to form the bend.

Next is priming, dimpling, and riveting on various doublers and stiffeners to the bottom skins. Everything here was back riveted and straight forward.

The step brackets are attached at an angle, and were a bit of a challenge to set some of the rivets, particularly the AD4- rivets in the corner. Here the tiny tungsten bar with angled sides came in handy, but still a challenge to drive and buck.

The mid-rib gear brackets are each attached with 3 AD 4- rivets that need to be aligned perpendicular to the bottom skin. I used a rafter square here, and checked/adjusted after every rivet.

When starting to attach the aft ribs, it was easier to set the whole assembly vertically by clamping the spar flat against a table or pair of sawhorses. This made the aft skin fairly floppy to start as there was nothing to support it, so I just clamped a piece of wood to it to keep it flat while attaching ribs. This put everything in a fairly accessible position to rivet. I ended up doing much of this single-handed, but the rivets near the center definitely require two people.

Finally get to break out the pretty gold anodized center spar sections! Everything here was pretty basic given that there was easy access to rivet components onto the spar. The only place it got a bit tight is in trying to tighten and torque the bolts for the bearing bracket. There's a screw head on the bolt, and not a lot of room to squeeze in there unless you have a stubby screwdriver.

Lastly for reaming the bolt holes for the rear spar brackets, I drilled a block of wood with the final size dimensions using a drill press and clamped it to the part as a guide to keep the reamer perpendicular.

Closing up the fuel tanks went relatively smoothly after a little planning, and watching over the video that Vans produced several times. I found that I could only do about 2 ribs in a row to stay within the working period of the proseal, which included mixing the proseal, apply it to a rib, get the rib seated and cleco'd into the tank, rivet the rib, and doing the final fillet / smoothing around edges, and sealing off the shop heads.

There were two places in particular that gave me problems that I would probably have done differently if I were to do it again. The first was the inboard external ribs that butt against the tank attach bracket. This area has a lot of mating parts, and on both tanks I noticed a small leak due to insufficient sealant between the skin and the bracket flange. Simply tracing a heavy fillet between the skin and the mating parts was insufficient for sealing this area. I ended up having to drill out many of the rivets in this area, and get creative trying to press sealant under the skin to get it sealed. In hindsight I would have just slathered all the flanges in sealant and dealt with the resulting mess. The other problem area was the float sensor. This is attached with 5 screws, and I'm still unsure if I need to seal the threads of the screws, or can seal it at the head - although this can easily be addressed afterwards and doesn't need any disassembly of the tank.

Getting the back bulkhead in place went a bit better than expected. Using the semco gun helped quite a lot here, and laying an even continuous bead along the entire flange was actually quite easy.

Lastly the nervous moment of leak testing the tanks. It took several iterations of trying to tie the balloon to the fitting in a way that didn't leak right where the balloon attaches. I found that a tight rubber band seemed to do the trick. I did find a tiny pinhole at the fuel drain fitting in one tank, and a small leak in both tanks in the exact same location at the point the tank attach brackets mate with the skin. Everything was fixed, and both tanks were able to hold pressure without any noticeable change in the balloon size for at least 2 weeks. Fingers crossed that means they're leak free and robust.