What is the best way to attach hinges with a gapseal to prevent control flutter.Lee

Once the surface is hinged flex it to its maximum deflection and take some monokote or whatever you are using and iron a piece in the there.

I usually cut a 1/2"x30" piece of Monokote and then fold it in half (glue side out) and then iron it into the gap. 1/4 inch on the surface and 1/4 on the wing, fin or whatever you are sealing.

Works well.

on my Diamond Dust when I covered one side of the wing I used the same covering to stick the aileron on too, like above, bend it to the extreme down and stick it on. And reverse on other side. Works good and is 100% gapless.