Sometimes it goes easy and sometimes not but...the easiest way I've found to seal the hinge gaps has been:

1) clean the gap surfaces with denatured alcohol first.
2) cut a strip of monokote the length and width of the gap you wish to seal.
3) making certain you know which side of the strip is the backing, slowly work a crease the full length of the strip.
4) pull the control surface back as far as it will go firmly and tape in place.
5) pull off the backing and using a ruler in the crease, firmly press in the strip to the gap. Your hinges will keep it from pushing thru. Iron down one side full length. Fold ruler down to shield the side you just ironed down and iron down the other side.

It helps to dance around a bit until the full strip is across the gap and seated all the way down. Once done, set the ruler aside and run your iron full length of the gap again on both sides paying close attention to burnishing the edges in good. This will cause the material to shift a bit but usually it all straightens up very nice.

When it doesn't, peel it out and discard. Walk away, get a coke and relax. Go back and try again when you feel ready.

You don't have to seal the gaps but if you just make it a habit you'll never have to wonder if the plane would respond better if you sealed the gaps!

Regards,

Clay