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 :-(