I don't know about Polyspan, but this works great with silkspan.

I do a couple of coats of Polycrylic on the bare wood, then I wet the silkspan, wring it out so it's just damp, then stick it down with Polycrylic.  It has a tendency to want to expand in the Polycrylic, but starting with it damp makes the job easier.

I have no clue how Polyspan would work.

I have used water based polyurethane on bare wood with no problems, even the flat surfaces like stab/elevator, flaps and rudders.   Yes if you put it on one side first they will start to curve a bit, but I put it on like doping wings.  It seals the wood and for me makes a good base for dope after sanding all the fuzzies down.   Then I go with a few coats of dope to have something to adhere the poly span(now Poly-T if you order from Stunt Hanger Hobby) with.  I put it down dry pulling as many wrinkles out as I can.   Once all surfaces are covered I carefully go over them with the heat gun.   I have also discovered on fuselages and flat surfaces that a iron will take out 99% of wrinkles that have formed from trying to get the Poly-Span to lay down while doping it down.    Use the iron after dope is dry.   By the way the polyurethane was a mistake at the time.  I thought I had opened a can of poly-acrylic clear.  It still worked for my application though.

I decided to reply to this instead of a new topic...
Is the stuff that comes in a SIG kit box silkspan? I tried brushing on minwax polycrilic (which i use exclusively now as my clear base) but i could not for the life of me get it to stick down the silkspan... I brushed on the polycrillic, stretched on the damp silkspan then brushed around the edges. When it dried, the silk tightened up a little and pulled right off the edges. I started over with another method and it worked fine. What did i do wrong?

Ok. That makes sense. I'll just do it the old way. It's too easy of a job to complicate it with experimentation.
What you say Tim, is exactly what it felt like.

I'm building a Blackhawk Fokker right now using Polyspan and Polycrylic (Minwax). I've had luck so far using a glue stick to tack it down. Be sure to get the grain in the right direction. I also brushed a coat of PC on before covering to seal the wood. Brush on the PC around the perimeter and ribs and allow to dry, then use the heat gun on low.PC will not shrink the polyspan as you know. I have been shrinking it fully after the first coat.