Joining batting to make larger piece
I am a beginning quilter and, as my skills have increased, so have the sizes of my quilts. But I have ended up with pieces of batting that are too small for my current quilts, but too large to throw away. Or use in pot holders or other such things--36 by 45 inches, for example--as suggested by some of you in another thread about batting scraps. One person suggested that there are tapes to iron on to join batting. Another, to zigzag stitch the pieces together. I have stitched batting together (as if they were normal fabric) for test/practice pieces and it leaves a lumpy seam, so I wouldn't do that in a quilt.
1) Where can I get such tape? What's it called?
2) I've never zigzag stitched. Do I overlay the batting edges or just lay them close together? What makes them stay close together?
3) Are there other solutions I've missed?