Is polyspan the same thing as Sig Koverall or is it lighter? I think they are both polyester cloth just wondering about the weight.
Polyspan is more like a paper made from polyester fibers. Sig Koverall is definitely cloth: it's just a simple weave.
Also, is it worth pursuing Polyspan lite because, I've heard it take more dope to fill and comes out just as heavy.
I dunno -- but it may be worthwhile trying something like Esaki tissue over the stuff. Esaki is almost air tight as it comes (it can be hard to get dope to penetrate for adhesion at times, in fact). So it should seal the Polyspan, and provide some complementary strength.
Lastly, can you apply polyspan with Balsa Rite? Should I cut the balsarite 50/50 with thinner like i do for monokote or use full strength?
I don't recall seeing an expert on this forum talk about using anything but dope -- nitrate dope, at that. I suspect that Balsarite would work fine to stick it down, but I don't know if it'd stick it down in a 20-point finish sort of way.
If you try, use the Balsarite that's made to stick down material with no adhesive, not something that's just supposed to help Monocoat along. I have some stuff that was sold as "Polyspan" by Hobby Lobby but which isn't exactly like the "Polyspan" that was sold by CLC (or RCM -- it's been a while). It looks similar, and came with stuff called "Balsa-Loc" that looks & smells like Elmer's white glue, and does a good job as a brush-on, heat-activated adhesive.