I save myself the grief and purchase the iron "tape" about 2 inches wide that is made to join the pieces smoothly. I have used it on several quilts small wall hangings to full sized with not a single problem. You just butt the edges of the batting together, lay the tape on the seam and gently press as directed, Quicker than sewing and not that costly per quilt. Love the stuff. Wish I could remember the brand name, but I am sure there are several.