I'd like to suggest that any new builders out there that don't have experience with drilling, riveting and general precision that they do NOT start with the main spars. Start with the rear spars that only have 1 capstrip on each side and are subsequently easier to assemble. This way you will also waste less material and man hours if you screw it up.
I have got through both main spars, but will more than likely rebuild the first spar that I completed. This is a big job, but I am now comfortable that it will go far more efficiently. I have finished the second spar and am much happier with the quality of the work.
Spar splice plate with flush rivets holding the filler plate in place.
