I have successfully used a LO heat trim iron when applying Ultracote trim to an Ultracote base. Technique differs if base film is over sheeting or an open area.

If you are trimming over solid sheeting you must go very slowly. Be very careful on the amount of pressure you apply as you may dent the sheeting. Like others have said holding the trim at 45 degrees to the base and move the iron back-n-forth working towards the lifted section. Make sure you add heat to only 1/8" of new trim. Any more than that and you will trap gas. Also, start at the middle of a trim piece and arc out from the center until you reach the end work your way to the end of the trim. After cooling go back with a new #11 blade and slide it into any bubbles from the side - not down vertically like a pin stick - and work the iron towards the slit.

If trimming over "no" hard backing the base covering will stretch a bit (from pressure - not from the LO heat). You can push down gently when you heat the trim. It will turn color (darkens) as the trim is heated. Again, only add 1/8" of unheated trim on each sweap over the trim, arcing from the center out, pushing any gas out to the sides.

Trimming over open backing progresses much faster than trimming over sheeting and with quite a bit fewer bubbles.

Good luck and show us your results!

I'll post some pics of trim covering after my check disk finishes with my RChard drive.