YLI Wonder Monofilament Thread   - Clear or Smoke 
 
The manufacturer says:  "The top thread for machine quilting .004 1500 yd / 1375 m. The very finest monofilament thread available. Use in the bobbin and needle for a really blind hem. Can be used in sergers or conventional machines." 
 
Londa says:  "My favorite monofilament thread - nylon.  Cross-wound spool so can use it both on horizontal and vertical thread delivery spindles.  1500 yards.  VERY fine at .004 .  I use clear on light colors, and smoke on dark colors."  
 
This item is now discontinued at my supplier, so if the Smoke shows sold out - on the  go to this product:  https://www.londas-sewing.com/product-page/monopoly-smoke-from-superior-threads
 
 
I use for couching yarns at 3.0 long and just wide enough to go over the yarn.  I use only in the needle, and lower the upper tension however much is needed so that I don't see the regular thread in the bobbin peaking at each side of the zigzag."