We tend to think that internal seals (and external tape) completely
eliminate any cross flow from the lower surface (hi pressure) to the
upper surface (lo pressure), but does it? Look at both ends of your
flaps or ailerons; The internal seal stops, so does the external tape.
It must or you couldn't get full down flap and full up aileron, at the
same time. The other end of these control surfaces are open also. What
does this mean? Air can, and will move laterally along the control
surface until it reaches the end and then move vertically and get to
the upper surface. We have just experienced cross flow.

The best we can do is slow it down as much as possible. I use properly
fitted mylar on both surfaces and control surface wipers (felt) at both
ends of the controls. Works for me. I also fill the flap/aileron well
at each end with foam rubber in an attempt to stop this vertical
movement of air.
JJ