Serge,
My experience with silkspan has been that the light grade requires a lot of dope (~8 coats) to fill ready for color, heavy grade requires the most because it absorbs more (~9 ish coats) and medium about 6 coats. On my last ship with a wing area around 450 sq's it added about 1/8 oz per coat of clear Sig dope 50/50 after 1 hr dry time (will lose a little more after full gas off).

You probably know this but just for anyone just starting with dope - resist the temptation to use straight from the can strength dope. Uncut dope takes forever to dry fully and will keep the covering soft for weeks (I did this and it took two weeks to fully dry). You can go down some on the thinner but in my experience not less than 40%. 

One other tip if you use Sig dope - use the Super Coat clear not the Lite Coat until the surface is mostly filled and a little shiny. The Lite coat does not shrink up to get the covering tight (this is for silk also). If as you get the third and forth coats on the covering doesn't shrink tight, give it more dry time and keep going, around the 6th coat it will re-tighten. Once it re tightens you can do the color and trim with the Lite coat.

Best,     DennisT

Ty, I'm very sure you sent me this good stuff; it was several years ago. You've always been very generous, and I've appreciated it. I have half the wing covered now, but only after difficulties I'd never experienced before with both silkspan and the butyrate dope that I've been using right along. I may post some pictures and descriptions when I've finished the job. I'm breaking for a couple more days to do the club newsletter now.

I decided to use what I thought was closest thing I had to good medium silkspan. So a lot of new stuff that had annoyed me on the original P-Force build got laid aside. I had bought some Sig plyspan (not polyspan) 11 years ago, because it had been in the hobby shop for some years (they were just catering to RC and plastic coverings). Plyspan came in several colors, and I purchased blue and white. Just after I used the blue, Ted posted about difficulties with his plyspan splitting and fading. Mine didn't split, but it did fade. So I thought that perhaps the dye was acidic, and this time I used my white (undyed?) stuff, which looked a lot like the old silkspan. I had much trouble getting it to stick; the next morning, I found that the plyspan had come loose in three places. It took a long time and a big mess to fix that seemingly simple thing. I've never had a wrinkle in a silkspanned wing in my life before this, but I had to remove some. it's fixed now.

SO... I'm going to experiment by using the same dope to attach the heavy grade, old (real) silkspan on the left side and see whether I have the same difficulties. The heavier stuff is quite difficult to tear, even across the grain; so I'm confident at least in it. I'll check weights and numbers of coats needed too. At least I ought to learn something from this.

I do have newer medium silkspan that I dislike. Is your medium material anything like the old silkspan? One picture of where I finally am now. Thanks again.

SK