Excuse me, folks, but I've been using Monokote for 28 years and I have yet to see a grain in the material. It is milar and, to the best of my knowledge, has no grain. You can put it on any direction with the same results. As far as sag goes, on an open bay wing, sag is normal, and as was said by 4*60, is found on full sized fabric covered aircraft. So, when using Monokote, tack it to the center section, pull it tight and tack it to the tip. Tack it to the leading edge at the middle of the span, pull it tight and tack it to the trailing edge. Keep pulling it tight as you continue to tack it all around the wing. When you've ironed it to all the edges, use the gun and it will look beautiful. The sag will be there, but not very pronounced.