I'd going to disagree a bit with Minn on the sealing bit. While it is true that a sealed hinge line is not immune to flutter, and there is no guarantee that sealing will not make the issue worse, in general, once source of the turbulent flow is from an excessive hinge gap. In addition, sealing, because of the less chaotic flow, makes for a more effective and precise control surface, so sealing is just good construction technique in my book.

You need to be careful and know what you are doing if you mass balance. Done improperly, the mass balance will tear off the control surface. The mass balance vibration can get 180 degrees out of phase with the control surface, and due to construction methods and materials we use in the models, it will quickly destroy the control surface. I know of one 40% 3D ship that was destroyed becase of the mass balancers.