OK
I usually overlap my seems about 1/4" to 3/8" as I make a skin.
( is what I call a section of mylar film covering that I have welded together on glass) I use my trim iron to weld the seems together. When i apply it to the wing I have just done it exactly as I have with a solid piece of film and have had no trouble with the seems opening. You have covered the bottem of the wing first I assume? The seems stay just as you have welded them. Now I don"t mean you pour on the heat either. I use my iron for the tack down arround the edges as I pull it on tight. Then I use my heat gun. over and Over again. If you would like to see one of my planes that I did a Skin on, it is a Sumpthin Extra I can send you a picture.

The DiBO