On the rear spar tip reinforcement plate there are some areas that need to be machine countersunk, Some people modify their countersink to fit the area, I just used my deburing tool with the countersink bit in it. Drilled into wood to keep it from wandering and held the extension still to make a pretty nice countersink.
I also began priming the left rear but ran out of primer. I am using the Napa Rattle can primer for the spars anyway and really like the simplicity of it. Also, it seems to dry faster and stick better than the PPG stuff, but might weigh a bit more. The PPG stuff seemed to scratch pretty easily. I will probably go with a combination of primers on the wing but will remain consistent by section. Will end up with some pretty colors though. Gold main spar, grey rear spar, white ribs.. We'll see if I can get some other colors in there too.
The riveting went well as I used post-its and clecos to help make sure I leave the holes empty that are supposed to be riveted later. These holes remain open and get riveted in assembly with ribs and aileron attach brackets etc..
Below is the left rear spar complete and ready to start getting ribs attached..