I have never experienced flutter, but many years ago I had a Cub that had terrible aileron control even though the aileron deflection was substantial. One day at the field when I was bemoaning this fact, someone told me to seal the gap. I did. I then had to decrease my aileron throw to less then half of what it was and I still had much more control than before.

Flutter or no flutter, that made a believer out of me. I always seal ALL of my gaps now.