Hi Mike;
I don't have too much experience with polyspan and like what I see so far. This has been discussed at length several times on the forum, so you might want to search this out with a couple different key words. Going from memory, if you are sure you have the correct side down, make sure you have enough dope on the frame when applying the covering and sanded well before covering. When doping down the covering, try making the brushing dope a bit thicker, but no so thick that you won't soak up the build up from the frame. Think of coating all the fibers in the material from both sides. Don't sand between the first three coats or so when applying the filler coats, and when you do start sanding, maybe go to a finer paper than what you have been using, and just sand it a bit more. using a block will help keep the surface flat. If when you are doing your final sanding, and you get some fuzzies, I have always wondered what doping down a piece of light silk span on the spot would help, kinda like patching a hole. Tear a round patch, not cut with scissors, and dope in place. The rough edge helps the patch to blend in. Block sand this down and see if you can feather the edges in. The paper will give you something to fill and sand off and hopefully trap the fuzzies. I hope to do more covering in the near future and have a Twister covered with Polyspan that I need to finish up and hope to try some of these ideas. Zinc sterate (spelling?) is used as a filler for dope when doing buildups, and I wonder if a slightly thicker solution of that in clear dope applied to fuzzies would help fill them and make them easy to deal with. But in short, a little more dope and a bit finer paper with more sanding with a block may get you where you want to be.
Good luck and have fun,
Dan McEntee