Inserted nearly all the flush rivets, held them in place with rivet tape, and began squishing them in place.
These flush rivets are set better than the ones I did in the right spar. Practice helps. The trick is to make sure the rivets have NO wiggle in them when sitting on the bucking bar. While on the bucking bar, if they can wiggle when you try to move them with your finger, then they are not seated in the countersink, and pounding on them won't do anything but set them with a gap between the rivet head and the sides of the countersink.