If you want to solve a looseness problem with a heat gun, choose only shrink tubing for electric wire. Pick your fights.

The problem with iron-on covering that develops loose streaks is not that it hasn't been shrunk adequately. The problem is the lack of adhesion between the wood surface and the film. On my models, that usually means I left sanding dust on the wood, or some lubricant got on the wood.

If you can't ignore the wrinkles, peel the covering, prepare the wood properly, and iron on some new covering.

If you can ignore the wrinkles you'll find your ARF experience more enjoyable.

Best wishes,
Dave Olson