ORIGINAL: Rocketman_
I'm another one who no longer seals control surface hinge lines. In my decades of building and flying models of many types including control line, free flight and RC I have never found that sealing control surface gaps cures a flutter problem. Proper wing/aileron design will prevent flutter in the intended speed range of the model.
I have seen wing flutter destroy freeflight models that had NO ailerons or any other control surfaces.
I've built models that had no flutter until I modified them to take tricycle retract landing gear which pushed the speeds up well over 100mph. Sealing hinge lines didn't fix it, slowing it down did.
Now I fly 1/4 scale and 3D with a Sig CAP231EX, two 72-inch Katanas, three U-CAN-Do-3D's and throw in a Pizzaz and a Somethin' Extra. Yep, none of them are sealed and they don't flutter. A bunch of other models that I have do not flutter either.
I'm just relating my experiences and what works for me. I'm not insisting that you don't seal but I don't think you need to be panicked into sealing by some who are only guessing that sealing works.
On the subject of control surface effectiveness, I don't believe I've seen a 3D model with sluggish controls. Its funny to hear that some folks use sealing to make their controls more effective then they use computer transmitter dual rates and high percentages of exponential to reduce that effectiveness.
Sealing a hinge gap won't prevent flutter. Sloppy control rods, play and cheap or not enough servos create this condition.
The only thing sealing the gaps buys you is a more crisp control response and much better near stall speed wing loading. I had a P-51 tip stalling on my without sealed gaps, I sealed the gaps and no more tip stalls.
Don't do it if you don't want to - it purely is optional.. but it does make a diff. I have yet to see a competitive pattern or IMAC ship that does not have gap sealing of some sort.
DP