Wayne, your comments are appreciated. The reason I sanded most of the glass and filler off is that the control surface was not going to balance the way it was- as built by others. I figured that the only way was to remove the offending (too heavy) build-up and re-glass carefully. But, you have pointed out something I have not checked. The balancing rod may not be the appropriate distance from the hinge point. Just a little would make a big difference. I'll check that out.
I never intended to sand it down and then used it in that condition.
What I'll do now, thanks to your input, is check the rod location, sand down to the foam, and then reglass as per plans.
Thanks for your help. Your comments have been extremely useful now & in the past in preparing my project correctly.