To add just a tad to what Mike just said
Close examination of what my wife bought cheap (Jo Ann's) and the RSM Polyspan:
Jo Ann's version does not (to me) have a distinguishable top or bottom..... I assume the RSM Polyspan has been "fixed" with something like fabric sizing on one face... I asume this sizing significantly heps with how much dope is needed to fill the weave
I made a fairly small open bay test panel and used both fabrics with my typical doping method...
Balsa bare wood Randolf full strength(FS) Nitrate 3 coats, sand, 1 wet coat FS Butyrate, set poly, very thin 70/30 B dope on perimeter.. dry...heat tighten... fill with 50/50 Butyrate fully drying each coat looking of full fill and a smooth feel
The "treated" RSM poly closes with one less coat of Randolph Taunting Dope....to MY eye and finger feel
Once doped, I weighed each and there is not enough difference in my notably small 4"x6" 2 bay trial...extrapolated to a 60 ~70 inch wing the weight gain is probably a game changer
If I did not already have enough RSM Poly for all the models I could possibly build the rest of my life I would be real tempted to switch to the much cheaper Jo Ann's product
I might even ask my wife to Iron in "Fix Sizing" spray to one side
BUT again the Cost (for me ((not too frugal)) ) is the RSM stuff comes in a form factor that is easy to cut and not waste... I would need to build a LOT of models to consider the meager aggravation of the Jo Ann's Hancock cheaper but nearly the same product
On the Other hand...IF you are a newbie on a severe hobby budget and wanting to built 5~10 models from your youth and do them sort of old skool... the Jo Ann's product WILL work, and be significantly cheaper with a slightly higher aggravation to cut and set
I am not yet ready to show my funky tries at Larry Rengers method
BUT I personally am happy with the result...just have to perfect with MY technique
NOT sure what the hell I am talking about...??
Larry R suggested covering the wing with Poly or silk, or tissue ---light weight covering and NO dope......Just final cover with Super Light Covering (SLC) from Phil Cartier...and call it done!...
I now have done two wings this way and it is faster, easier, much lighter BUT my personal technique still leaves visual NOT STUCK down SLC over the Poly....grrrr
BUT in the areas where I don't have issues...really is strong and looks fantastic
BEST advice
Support a modeling friend and get the very good and easy to work Polyspan from RSM... The Stuff is NOT too much more cost...
BUT, love supporting them as I do... Dharma and the other places have good Silk, so much cheaper, I can not recommend the Brodak, RSM, or other cottage suppliers
Dope ---same advice...I spent first few years returning to this hobby using Sig and Brodak dopes
I Found a REAL aircraft supply in Dallas and bought gallons of thinner and dope..YES ....still pricy, but cost per gallon a LOT cheaper than pints or quarts from our Sig and Brodak friends
I need to address Herman Green's real first question
Yes sir... the Poly fabric can be found with a wide variety of weights and closed or open weaves,,,, just like Silk span
If you suspect you got super light fabric..it is still usable but I highly recommend Larry Renger's method for 36 to 70 inch models...
I suspect for many .15 down to 1/2a models the lightweight poly will be just fine as is
BTW, I experimented and Preval sprayed MinWax H2O (water based Poly crylic) on a Brodak Lil Jumpin Bean I covered with questionable weight Poly off e-bay ((SEEMED real light and flimsy to me once applied)))... sanded smooth it took color rustoleum fine and so far holding up to my abusive flying