Crashem:

This is extremely simplified, but think of a whistle. The smaller the whistle (gap between the control surface and its mount) the higher the pitch.

By decreasing the gap we get the whistle "High" enough that it does not have any effect at our air speeds. Conversely, with a wide gap (low pitched whistle) we can tear the control surface off at common air speeds.

By sealing the gap we remove the "Whistle" entirely. Sealing the gap works, it should always be done if there is any worry about flutter. It also makes the control surface more effective, and it's no trouble to do.

My twin had gaps, wide. When it came apart in the air I said "I Tried."

Bill.

PS: Leaving the trailing edge of the surface blunt also decreases the tendency to flutter. In other words, don't taper your ailerons. wr