Sealing the hinge gaps is always a good idea, and for what it's worth your hinge gaps look pretty good. You can use clear monokote if you want to be really thorough, but box tape will work just as well.

In either case, just put the control surface at maximum deflection before sealing so there is enough slack for the surface to move. Full "up" when sealing the bottom side of the control surface, full "down" when sealing the top. The slack will make it look bad in any case, so you may as well just use box tape.