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<pre>If you stretch it between your fingertips till it snaps, it frays. You can
then pull back the frayed ends as far as you want.
My Otto was done this way:<
Noooooooooooooooooooooo!!!!!!!!!!!!!!!!! Told you at SMW, if you do that then the stuff will perish. Might take longer in your neck of the woods, but everything I've done it to went after a maximum of 2 years.
To the original question, I don't know what the heavier stuff is good for, but I wouldn't use the fine on anything smaller than 1/48th (I've tried it on 1/72nd and agreee with Steve, it is too thick) . I use charcoal/black/whatever he calls it now for general use, but only because he doesn't do silvery stuff. For French (the CSM Caudron G.3) I used the blue and think it looks alright.
The only real problem with the stuff is that the thickness can vary, so you can waste a fair bit looking for consistent lengths. Okay, it's more flat than round section, but so long as you don't let it twist that's hard to see on the finished item.