At a recent seminar on ship rigging, a guy showed some super fine thread (3 mil), and I bought some. I have been looking for a finer thread, since most of my threads look too large for smaller scale stuff. This stuff is almost invisible. I am using it for some rigging on a WW1 biplane, and I am finding some of the problems with very fine rigging thread!
Visibility is okay if it is in bright light, but on a biplane it is sometimes hard to avoid some of the area between the wings being in shadow. Then, the thread disappears!
Also, I find my tweezers sometimes cannot grab a thread that fine. The points are uneven, and seem to have flaws and unevenness on the order of the thread diameter. I hold a doubled piece of sandpaper with the tweezers and draw the sandpaper through. That seems to help, but sometimes still cannot hold the thread!
It looks very nice when I get a piece rigged, but sure is hard to work with 
Don, what scale bi-plane? Have you tried the E-Z Line brand? It comes colored so maybe it would be easier? I’ve used it for WW2 aircraft aerials and it sure beats stretched sprue. Here’s a link to the review:
Yeah, I have tried it but not real satisfied with it. It looks even bigger than my translucent black monofilament, which is same size as the EZ line I tried (both .007). This new stuff is only half that size, so looks great- barely noticable. Nothing is as easy to work with as real thread, but I just cannot find any real thread small enough for normal modeling scales. I guess it is one of those fundamental tradeoffs of nature. Whatever looks best will be the hardest to work with.
BTW, I am working on an Eduard 1:48 dH-2. Have rigging finished now, but it was sure work! Now finishing up with reserve gas tank and gas lines. Oh, one other thing about that 3 mil thread- you must hold your breath when working with it. Normal breath, even from nostrils, makes it whip around like a rope in a hurricane!
HaHa, Don, I always hold my breath when doing rigging!