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.