Prism99

I use YLI nylon monofilament for this and never have any problems. I have tried Invisafil and polyester clear threads; none are as invisible as the nylon monofilament. Brand name is important in nylon monofilament, which is why I use YLI. It is as fine as baby hair.

There are some tricks you need to know to use nylon monofilament without problems. (1) For one thing it is stretchy, so it is important to lower your upper tension. Test. I drop mine down from a 5 to a 3 or so. (2) How you wind the bobbin is important. Because of the stretch, it's important to wind the bobbin slowly. Never overfill the bobbin. If you have metal bobbins, you can fill about 3/4ths full. If you have plastic bobbins, do not fill more than 2/3rds. This is because the stretch in the thread can cause the bobbin to distort.

Edit: That's a great website posted above. I forgot to mention that I use a cover on my cone of YLI and also use a thread stand -- to make sure that the thread is feeding evenly from the top of the cone into my machine. I *love* the new Superior thread stand for this and other applications (such as winding bobbins on my standalone bobbin winder).

Edit 2: I should add that I have tried the Superior MonoPoly. It is thicker and stiffer than my YLI nylon monofilament, and shows more. I have ironed over the YLI thread with a hot Rowenta iron just to test, and it has not been bothered by the heat.