Lots of people are using polyurethane to fill the weave. This works, but how tight the fabric gets is totally dependent on how tight you shrink it before filling the weave. With nitrate dope, it continue to shrink over time and actually helps maintain the fabric in a taught condition. This is why it is still done on fullscale aircraft. I haven't tried polyurethane myself, as I still prefer nitrate dope (stink and all). I think maybe polyurethane, when dry, is stiffer than dope, and the fabric is tight partly because of this. Either RCKen, or Minnflyer, I can't remember which, has a good tutorial on finishing Koverall. Whichever way you go, I think it's (Koverall) the best stuff out there.