What I like to do is use both razor blades and a razor saw. Because the razor saw has a lip on the back edge it's tough to use it by itself, but since you usually have to remove about 3/8" of material (then add 1/8 to the trailing edge of the wing and 1/4" to the leading edge of the aileron) I usually cut the 3/8 from the top and bottom sheeting with a razor blade and remove the sheeting on both top and bottom, then cut through the ribs with a saw. Then just sand everything even.