John,
I was fighting a bad twist in a newly covered wing. I tried heat from a heat gun, jugged it up and it did not work. Finally I went back the the age old method of steam from a kettle. This was desperation move that I really didn't have much hope for but I tried it anyway. Damn, if it didn't work. There is something about steam getting the heat deep into the wood that works (I don't think any of the moisture gets through the dope, just gets the the heat in). I would try the steam to get a more permeant set and prevent the dope from relaxing and re-warping once the ship sits in the sun.
Best, DennisT