geh3 makes a good point about the film sliding off the peripheral frame when you don't wrap it around a corner and seal it down well. It used to be that Monokote would stick tight without needing to wrap around the corner, much better than Ultracote in that regard. There are some sections where you don't have a corner to go around, and that was one reason I always preferred Monokote over Ultracote. For instance, where you have a stick-built, unsheeted horzontal stab with the covering removed from the center to allow gluing to the fuselage framing. If the covering has only a narrow edge to cling to, the Ultracote will slide off when you apply heat to tighten it up. As it stands now, my solution is to balsa sheet such areas, and adhere the Ultracote to the full sheeted surface. I just warm the covering up to sticking temperature, then wipe it down with a soft glove.