You said that it is SHEET balsa?? Is the trialing adge not tappered at all?? If they are not tappered yet I draw a line on one edge where it will mate up to the wing or tail and kind of eye ball the angle and use a #5 wood plane made by Stanley. I picked it up on Ebay for $12.00 plus shipping. It is longer then the MA razor plane and when the blade is sharp cuts like you are goun through butter.

If you are asking about how to angle the leading edge of the control surface, then I use a 6X48 belt sander and then touch it up with my plane.

Dru.