We have published this before but since it's one of the most common problems we get calls and letters about, here it is again! First of all, your ailerons, elevators and rudders can be very thoroughly sanded, far more so than the rest of the aircraft. Use a blue foam (Styrofoam) block, sized to fit your hand, and a half sheet of 40-grit sandpaper. Sand vigorously the top and bottom skins of the control surfaces, particularly toward the trailing edges. You can safely sand off up to 50% of the top ply of UND - this leaves one and a half plies of UND - more than adequate for control surfaces.
What it does is reduce the weight of these parts considerably, especially aft of the hinge, which makes it much easier to balance and, more important, since it is now very smooth it takes much less fill and paint to finish the part, making it easier to balance. Using this method, and assuming reasonably good workmanship, it should be easy to balance your elevators. Elevators absolutely must be balanced per the plans criteria or they will flutter.
This means they must balance after finish. Ailerons are not as critical due to the much stiffer wing they are hinged to, but even though we have not had a single case of aileron flutter reported, you should still be sure to balance them within the plans criteria. If, after sanding them thoroughly as called out here and checking to be certain that the mass balance is correctly positioned relative to the hinge, they still don't balance, the best method of adding mass balance weight is to go to your nearest golf pro shop and purchase a roll or two of the soft lead ribbon used by pros to weight the heads of their clubs. This is a 3M product and consists of a roll about 112" wide of lead ribbon with a sticky back.
Stick it on top of your existing steel rod mass balance, as far forward as possible without increasing the chord of the ailerons. Stick it on the full span. Use as many layers as it takes to balance within the criteria, then lay up one ply of BID over the lead to permanently attach it to the aileron. EZ type rudders do not require balancing, however they can benefit from a thorough sanding because it will take less fill and paint to finish and therefore, they will be lighter. As far aft on the aircraft as the rudders are, excess weight here is hard to take care of. This is the method we have used for many years here at RAF and it works well. In about every case, the sanding alone will balance the ailerons and elevators without any additional lead. At least, this has been our experience.