Monokote over itself or Ultrakote isn't too bad if the area isn't extremely large (say about 1-2 inches max wide) and you can "roll" it on slowly so the air can escape as you go. Use a low temp iron to start with. Then you can raise the temp for a final seal; just don't let the gasses start building up.

Otherwise you could try the "windex" method which many swear by. I haven't tried that yet so can't say.