Anything you put on the balsa and then sand will screw you up. They sand at different rates. I use a metal sander now, (great planes) and it works well, but don't sand too much. You will make a mess. You will never get it absolutely perfect. Just get it close and cover it. Noone will ever see the imperfections. If they dip, they will not show under the monokote. If they stick up, sand them off so they don't or they will be seen.