Completed prepping the mass balance weights and epoxied and riveted them in place. Note - I installed the bottom side rivets only; four pulled monel rivets and one button head solid rivet (see picture). Reason: the top side curve of the mass balance weldment didn't match the curve of the flaperon. The gap was fairly minor but I was afraid that a pull rivet might deform the skin as it pulled if it should pull the skin up to the weldment. So, I made sure that epoxy filled what little void there was - once hardened, it will keep the skin from being deformed by the rivet being pulled.