The plans were not particularly clear on this, and I only discovered exactly which flanges should be dimpled in the top wing skin section. In short, do not dimple upper flange of the inboard 4 ribs! The doublers for the skin to reinforce the section that can be walked on are too think to dimple and will be machine countersunk instead.

Riveting the rear spar to the wing ribs was accomplished mostly with a squeezer. There were a few places where it was really challenging to fit a yoke without interfering with another rivet shop head or a flange. I found that I ended up cutting a small groove into a few shop heads, caused by the yoke dragging a corner against it. These didn't seem too deep, however, so I simply smoothed them out with a file.

The hardest part of the spar for me was the new inboard aileron bracket from the service bulletin. The rivets along the outboard flange of the bracket couldn't be squeezed because my squeezer would interfere with the bracket itself and couldn't be square. For these rivets I had to use the gun and bucking bar. For the inboard flange of the bracket, the neighboring shop heads interfered with even on offset rivet set. For these, I had to flip the rivets around so that the manufactured heads were on the inside of the spar. I couldn't quite square these up, and the shop heads ended up being

One challenge I've had is riveting flanges against a webbing or similar cases where the outer surface wants to lift up to create a gap between the two pieces of material. I made a few observations here and have found an approach that seems to work pretty well in a variety of circumstances.

Back-riveting

When able to back-rivet, I've found this to be the easiest as the back-riveting tool has a strong spring that compresses the pieces together before riveting.

Conventional riveting

When using the rivet gun and bucking bar, unless you press the sheets together with your bucking bar hand a gap will often form between the sheets. This has caused me a lot of early frustration, as once this happens the sheets become slightly deformed and even after drilling out the rivet it becomes even harder to get them compressed flush without a gap.

The technique that I've found easiest is to get a piece of rubber that has a hole punched in it just large enough to hang on the end of an unset rivet. You have to be careful on the thickness of the rubber; if it's too thick the bucking bar may not make contact with the rivet shop end. If the rivet is just slightly set with the rubber to compress the pieces, they will hold together. Then the rubber can be removed and the rivet fully set normally.

Squeezing

When rivets can be squeezed, I've found having the rubber insert works well here as well, but doesn't need to be removed to fully set the rivet. I've also found that the rubber pad tends to square up the squeezer to be orthogonal with the surface.

While not immediately obvious when starting the aileron hinge brackets I found that the parts were missing from my inventory for the inboard brackets. After digging a bit, I found that my kit included the service bulletin 16-03-28 subkit that address potential cracking problems with the original brackets. The new brackets are considerably heavier with a piece of angle aluminum instead of a folded flange, as well as two doublers with their own flanges on the backside.

When match drilling the J-channels against the wing spar, the plans call out for omitting 4 holes that are near the nutplates on the edge of the lower flange (red arrows in the figure). These were rather hard to see in the drawing, and I totally drilled them, only to realize that once you match drill against the upper flange you'll get 2 holes that are almost but not quite on top of each other. I'll probably consider placing a doubler over those locations once I get to attaching them to the skins, and if that won't work I'll redrill a new J-channel unfortunately.

For countersinking the fuel tank attach nutplates, I first attempted to follow the recommendations from Vans to use a #30 pilot countersink using the nutplate itself as a guide. I used a standard 3-flute countersink for this, and I can't say it works very well. The pilot still has enough play to cause the countersink to chatter around and without securing the cage this resulted in a really rough countersinking. Doing some research there were seemingly two viable alternatives:

• Many other builders seem to have manufactured guides that could be cleco'd to the rivet holes of the nutplate and provide a perfect pilot to center the countersink. While this seems the most precise, I had already riveted the nutplates in place.
• The simpler alternative was to use a single-flute countersink that cuts better and doesn't chatter nearly as much. I decided to try this path first since it was frankly simpler, and ordered a #30 cutter from Cleveland aircraft tool. I think the results turned out pretty well here, a comparison is in the above photo next to their respective countersinks.

The plans call for a max diameter of the inner and outer countersink holes. I found this pretty hard to measure, and didn't have a piece of dimpled skin to compare, but got as close to the maximum number with a simple ruler as I could without going over.

Finally, the plans call to spot prime all the countersinks. I've read a whole lot of approaches here from creating a stencil and spraying to using a q-tip. I really wasn't in the mood to spray anything as I only have an LPHV gun and don't have primer in rattle cans, so I chose the q-tip option. I mixed up some P60G2 and dabbed each countersink hole with the q-tip. I can't say the results look beautiful from a paint evenness perspective, but I think it got the job done and didn't take too long.