Well, I usually get the polyspan as tight as I can with a heat gun, hopeing for no runs. Then I use taughtening dope  only on the area above the bays.  Usually two coats, the I go to non taughtening dope as by now I am covering the entire wing. Polyspan will NOT shrink from the dope, the dope shrinks, so it isn't really necessary  on Polyspan. I am just using up my Nitrate dope.  I don't even use it to apply any type of covering anymore. 

I found it, taughtening Nitrate, never stops shrinking and wood grain keeps showing up after the plane has been painted.. Very frustrating having to go over spots all the time.