I read two interesting articles on pieced batting by Marje Rhine, technical pattern editor for American Quilter Magazine. She shares two methods - by hand and by machine.
Large pieces of batting can be pieced together by hand to use in bed-size quilts. A little preparation ensures that the pieced batting will stand up to normal wear and tear on a quilt. First lay the batting pieces on a rotary cutting mat, overlapping by about 4”, and rotary cut a gentle wavy line through both pieces.
Remove the small excess pieces, line up the pieces along the curve, and hand stitch cut edges together with large stitches. To make sure the stitching will hold, run a thread in each direction.
Here's a method for joining the pieces by Machine:
I needed a quick method for joining the pieces so they would lie flat without a bump, instantly giving away the secret that they had been pieced.
I tried butting straight edges of the pieces together then stitching together with a zigzag that catches both pieces. This worked OK but the batting tended to stretch as I sewed. (It also has that telltale little bump where the seam is.)
1, Layer the batting, right-sides together (or same-sides together) along the straight edge.
3. Open up the batting and pull slightly. The stitching should hold the length of the seam and almost disappear both visually and to the touch.
Here is a link to the articles:
I've used the zig-zag method and it does have a small bump. I'm going to give the blind hemstitch a try next time I need to piece batting!