Securing monofilament thread tails
What is the best way to tie off monofilament thread to keep it from unraveling?
As I think you've noticed it is virtually impossible to tie a traditional knot in monofilament thread and have it hold.
I use micro-stitches to hold the beginning and end of a line of quilting made with this clear thread.
If you are using a walking foot...
...set your stitch length to 25-30 stitches per inch(spi) and sew with these itty bitty stitches for about a quarter-inch or about 8-ish stitches. Then change to your regular quilting length and keep going.
A quarter-inch from the end of the line of quilting, again reduce your stitch length to 25-30 spi.
If at all possible I like to hide this micro-stitching in the ditch. Because each stitch is so short, it doesn't reflect the light as much. There is not as much space to 'glisten' as in the longer, regular length ones. By hiding them in the ditch you don't notice any change in the 'look' of the line.
If you are using a darning foot...
...there's no stitch length to set. You are in control of the length by how fast you move the quilt sandwich in conjunction with the speed you are running the sewing machine.
You're still looking for 25-30spi. With the machine running at a consistent speed, simply move the quilt sandwich slower to put done shorter stitches. Move it faster to make longer ones.
I've used this technique with several flannel quilts...ones that get washed pretty regularly during the winter. The threads have held without any breakage that I can find. (I do wash in cold and lay my quilts flat to dry.)
As for the bobbin, I use a 50wt 100% cotton, either Aurifil or Masterpiece. Presencia's cotton would work just fine, too, but I don't have any of that on hand. The point is you don't have to use monofilament in the bobbin. The cotton works just fine.
I love working with monofilament. YLI and SewArt (I believe YLI has acquired the SewArt brand, now) are my favorites. I hope with this technique that you'll love it, too!