Per Mark's advice, we kept them concave. We have an 8 foot piece of aluminum for a straight edge and used that to fill gaps with micro along the long axis of the elevator. We moved the straight edge down the elevator about 1.5 inches each time and marked the low spots (light nder the straight edge) with a marker making a line. We then drew a line around these horizontal lines to get the irregular area to fill. You can see some of these areas on the elevator in this photo. I agree that filling once and not trying to sneak up on it is best, but not for this particular chore of keeping the surface concave along the entire length of the elevator. Ours was really bad on the right trailing edge because of the way we clamped it. It rose up when it hit a rib and sagged between the ribs. This is because the ribs were not sanded down to make the thickness of the TE the same over the ribs as between the ribs! It also had a dent under each clamp because we did not use a piece of steel under the clamps. We did it right on the left elevator!!