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There is an iron-on "tape" that is sold in quilt shops for putting pieces of batting together. It is basically tricot interfacing that is already cut to about 2". I've bought the roll since I was in the shop, but interfacing would work.
I Quilt Therefore I Am
Pat
I just handed in a quilt to my LAQ and asked her about the iron-on tape. She said she would not accept batting that had been taped because she has tried it and found that she had problems with skipped stitches, broken threads, etc. when her needle passed over the taped areas. She went back to the old method of joining pieces together by hand or machine. She also said that she will no longer accept Mountain Mist batting. Said she used to have no problems with it but lately she has had problems with the batting separating into balls after multiple washings of the quilts. She believes the scrim is somehow dissolving after repeated washing in water. Some quilts are fine but others aren't. Of the last 4 quilts she used this on, she found 2 of them with this problem.
This is very interesting to hear from someone who should know what she is talking about. I used the lightweight interfacing and a wide zig zag on a couple charity quilts, because I wanted to see if I had any issues with the long arm going over the interfacing part. I had no problem at all. I don't use Mountain Mist any more either, because I had used it on some of the GKs baby quilts and noticed them getting all lumpy over time. I now only use Warm & Natural or Warm & White.
I use the iron-on heat tape as well that comes on a roll but cut it in half to 1 inch strips and it works just as well and the tape goes twice as far.
Like auntpiggylpm I use fusible interfacing, and have for years. It works great and is inexpensive.
just remember the fusible on the tape doesn't last. you must quilt over that seam well or just go ahead and zig zag it together instead of fusing.
All those products are fine, but I'd rather spend money on fabric. I simply butt the edges of batting scraps together and hand stitch, using a very large whip stitch. The actual quilting holds the pieces in place permanently. I've been doing this for nearly 30 years and never have had any problems with separation.