Using the monofilament thread in your bobbin for many projects is a good thing. When you use a color that matches the back of the quilt in the bobbin and monofilament on top, the thread from the bobbin sometimes shows through to the front of the quilt. This happens often when the machine quilting stitches get shorter. If the color on the back is dark and the fabric in one area on the top is light then that dark bobbin thread pops up a little and shows on to the front of the quilt. The opposite happens if there is light thread on the back and dark thread on the top and your top fabric is a light color.

If you have an older machine it may not "like" the monofilament in the bobbin, so you will have to use regular thread.

Try using a "busy" fabric for the back of your quilt. It helps hide the stitching, especially if you are changing colors of thread in the bobbin.

Another tip: When filling the bobbin with monofilament hold the end of the thread for a long time before cutting it off, other wise you might get a rat's nest...Also if you have a plastic bobbin, do not fill it all the way as the plastic heats up and won't come off the spindle till it cools. There is a little stretch in the monofilament thread and may pull a little and some brands of plastic bobbins may break.