i noticed the rudder isnt totally flat, it has a little cupping like it slightly concave.... how should i fix this? i was thinking about using dental floss from the tip of the rudder to the tip of the elevator....
I assume you mean the vertical stab is concave, not the rudder. Is the stab foam on the Slow Stick? If so, you should be able to bend it slightly past flat and it should spring back to flat when released.