Prism99

It was probably the wrong type. My favorite monofilament is YLI nylon monofilament. It is somewhere in the neighborhood of .003 or .004 in diameter, making it as thin and fine as baby hair. A lot of people these days prefer a polyester monofilament because it can stand higher heat than nylon and does not yellow, but I have not found one as fine as the YLI nylon. I have tested both polyester and nylon monofilament threads under my very hot Rowenta iron and found that the nylon will deform under the hottest heat while the polyester does not; however, since I never use my hot Rowenta iron on an already-quilted piece, this isn't a big consideration for me.

I have used monofilament in both needle and bobbin on my domestic machine for quilting -- no problems at all. You do have to be careful how you wind the bobbin. Nylon monofilament stretches, so you want to wind bobbins fairly slowly (so the thread does not stretch as it is loading onto the bobbin) and you want to underfill the bobbin slightly so that the edges of the bobbin do not deform from the pressure of the thread. You also need to loosen the upper tension quite a lot, so the monofilament does not stretch as it goes through the needle.

For any kind of free-motion quilting, I would recommend using monofilament in bobbin as well as needle because the tension changes slightly with the type of motion you use. This means that dots of bobbin thread are likely to appear on the top -- if not all the time, then at the times when the tension balance changes with changes in direction, speed, etc. I have to wonder if the shop owner has any valid reason for recommending not using monofilament in the bobbin; I haven't run across anything that would make me stop doing this!