Use a long strip of covering (whatever you used to build it, or if it's an ARF, whatever they used to build it). You can use clear or a color to exactly match so it does not look obvious. The length should be slightly longer than the gap you intend to hinge.

Make the width slightly smaller than the inside width (both sides) that you are going to fill.

Fold it in half lengthwise, exactly (or as close as you can get it) down the center. Then remove the backing. Place it in the groove with the surface flexed as far on the opposite side as you can get it, then fit the covering down so that it is completely flush with the gap and evenly spaced on both sides.

Use a small iron (there are both regular sized irons and a smaller trim size iron) and seal it starting at the middle and work your way out making sure to keep the heat even on both sides while pushing down into the hinge line to make sure it goes all the way into the gap. Do both sides (of each aileron.... remember, you started in the middle) then both wings the same.

It is not necessary to seal the elevator hinges nor the rudder hinges unless the gap is like an eighth of an inch or something..

You don't have to do the top and the bottom of the aileron hinge, by the way, just one side. You can probably guess that the bottom would probably be best so that any mistakes are not immediately obvious to a casual observer.

DS.