If you're using moisture, use light wipes along the side of the framer/s which you want to expand. Too much will soak the entire spar and defeat the purpose. One nice thing about iron-on covering, is that after enough intentionally applied twist and heat reshrink cycles, you can usually get a warped wing to finally stabilize. Of course it requires some overshoot, and sometimes a number of tweaks done over the course of some time, to finally get it to stay where you want it.