I just finished sealing the hinge lines on an ARF that I recovered using Ultracote.

Top or bottom? I let the covering scheme dictate whether I covered the top or bottom. In my case the bottom of the wings, ailerons, stab and elevators had the same base color - midnight blue. So, the aileron/wing gap and elevator/stab gaps were sealed with matching blue covering. The top scheme has yellow for the ailerons and elevators but the wing and stab trailing edges are covered in orange. The rudder and fin (starting at the bottom) are blue,orange and white. I used individual pieces to cover each section matching the lines as best I could. Of course, I already had an inventory of film scraps from the original covering making this selection easy.

If you don't have matching scraps grab some clear Monokote. Forget the light weight stuff because it's difficult to handle.

On tape? Yea, works but if you fly glow the exhaust spooge will get in under the adhesive and a mess will result.

Installation?

Cut a piece as long as the hinge gap (minus 1/8" or so) and 3/8 - 5/8 wide. Whatever width fits the "V" without overlapping the bottom of the surface too much. Pull the backing off the covering. Crease the covering into a long "V" with the adhesive to the outside.

Open up the seam to its fullest extent. Let's say for argument that you're covering the bottom seam. Remove the servo control rods. Open the control surface toward the top as wide as easily allowed and tape it open.

Position the seal trim into the hinge gap. Use an old credit card / hotel room card / piece of stiff card board and seat the bottom of the "V" into the seam. With your iron on low heat (enough to activate the glue) start applying the trim piece to one side of the seam. Do the whole length before doing the other side of the seam. Repeat on the other side. Turn the heat up to full shrink heat and go over the entire seal to make sure that the adhesive if FULLY activated. Roll any exposed trim over the trailing edge and control surface.

Repeat for the rest.

HTH