If you are going with polyspan, even though it will heat shrink some, I would still put at least a couple of coats of tautening dope on to fill and finish up with non-tautening. All nitrate might be OK if going over it with Rustoleum. I just don't know if those two are compatible at all in any form.
I still contend that there is no such thing as a "fast finish." Guys have been trying to find it for as long as there has been silk span and dope. The quickest finish I have found has been Allen Brickhaus' finish methos that involves iron on covering, spackle compound, finish cure epoxy and rustoleum or x-o-rust paint. The short version is a layer of watered down spackling compound, sand when dry, and then another coat and sanding. Then a layer of thinned finish cured epoxy, scraped and blotted off, then sanded. Apply a little more to hit light spots. Then primer and paint. Wings are covered with iron on covering first, then wrapped in foil for protection through the other process. The epoxy coats overlap over the iron on coverings. The iron on coverings is failr fast but still a lot of work to do well. He covered the whole process in detail ina couple of his construction articles, one being his semi scale Beguine P-51. It is worth checking out. I have done a few this way with decent results and I didn't do it exactly correct either.
Type at you later,
Dan McEntee