Be specific on heating your dacron. Calibrate your iron at home with a candy thermometer and the iron's rheostat setting at 225, 250, and 350 degrees F.

Heat the dacron in steps, stopping when you have it to your liking, but no tighter than 350 F. The more Aircraft Dacron is heated the more it shrinks to a point

When Aircraft Dacron is heated above 350, the dacron fibers start to melt, thus relaxing the fabric. There is no way to repair this damage. Hopefully this knowledge explains why a heat gun is not a suitable tool.

Aircraft Dacron will shrink 10-12% at 350 F.

Non-aircraft Dacron may already be preshrunk.

If the dacron relaxes after it is covered, try re-heating it. It has tightened back up for at least some.