Honored experts (except ET since he only plays with splinters)

I have been building SOF kayaks with 8OZ dacron from George Dyson.

Recently I have been less than happy with the tension in my skins.
Today is is cold (relatively for Texas) and my recent boat seems to be less tensioned than when I started painting.
I have looked at an aircraft tail covered with a typical system of coatings and it was really nicely tight - vastly better than mine.

An aircraft making friend told me that using a heat gun in stead of a calibrated iron could either over heat or underheat the cloth, both ending up with not as tight as possible.
I tried to get some scraps from two different guys, but it hasn't happened - to do an experiment.

Any suggestions or explanations?

I have used Minwax Spar varnish, Interlux boat paint, and Behr Oil based house paint, no significant difference. Always with a heat gun. My coverings typically have only very small bubbles and start out well stretched before shrinking.

Thanks

PS ET is welcomed to chime in here anyway, no one can stop him :)