I'm sure others much more expert than I will answer this, but you most certainly do NOT want to fill the concave portion of the elevator. It is an important part of the aerodynamic design, but I'm not sure of just what will happen if the elevator section were to be straight. Also, it is important that the round portion at the leading edge be flush or preferably "stand proud" of the stabilizer surface. This is especially important of the ailerons. I believe that is so that the boundary
layer attaches itself to the control surface. Most of the control, at least for small deflections happens when the air follows that rounded surface, not by being deflected by the trailing edge. Some builders have found this out the hard way when the aileron forces are very high and roll control is less responsive than it should be.
Gary Casey
ES #157
I have completed 90 % of the fuselage construction on my Legacy and have saved virtually
all of the finishing until now. I thought I’d start with the elevators to get some experience. The previous builder had completed the elevator construction which leaves me with the question: Do I need to fill in the concave shape to create a straight Elevator surface or just prime and sand what I have?
Stan Fields