When covering with Polyspan, it is important to put the right side up. If you candle it under a light, the shinier side goes up but often times this is hard to determine. The best way is to drag the Polyspan over a small piece of 600 grit sandpaper. The down side will want to grab the sandpaper and cause a lot of friction and resistance. The smooth side, the side you want up, will slide smoothly over the sandpaper.
When you put the wrong side up a dope it, you have a fuzzie problem on your hands that you can rectify, but it takes a lot time and irritates the heck out of you.
Mike