JJ,
I use monokote over ultracote all the time for hinge gaps. Holds fine. I use either clear monokote or I use left over material from covering jobs that match the wing and ailerons colors. Also, you can seal from the bottom or top. I always seal the bottom. If you seal the bottom and you fly off of a grass or dirt runway material can stick to the seal material and will be noticeable from the top over time.

I hope this helps

Mike