Here is how I fill in my control surfaces. Works well and no cracking after doing it on 3 RVs. Hides some rivets though and gives you that composite look so not for everyone.
Enjoy
https://www.youtube.com/watch?v=n0oSff-7hqY&feature=youtu.be
Enjoy
https://www.youtube.com/watch?v=n0oSff-7hqY&feature=youtu.be