Prism99

Regarding the seams, I was thinking of going over the existing piecing seams with a zigzag using polyester thread. You don't have to go over the corners or bulky spots; just over the straight seam portions where it is easy to do. This would reinforce the straight-stitch Aurifil seams and prevent the seams from opening even if the Aurifil thread happens to break. After some thought, I don't think you even need to use Bottom Line for this. Whatever polyester thread you use for the quilting would work fine in the seams. When zig-zagging, I would secure the poly thread in the seam allowance, then gradually taper the zig-zag so the zig is just to the left of the Aurifil and most of the zag is in the seam allowance. Basically this is covering the Aurifil thread with reinforcement and, if the Aurifil thread breaks, the poly thread will hold the seam together. A slight zigzag like that does not show enough on the right side of the top to matter. You wouldn't necessarily have to go over every single seam; just those that are easiest to do. One of your biggest problems is that Aurifil 50wt is a weak cotton thread that breaks easily, and you used a straight stitch with it. Reinforcing those seams with a polyester zigzag stitch will help a lot. Experiment on scraps to get a zigzag stitch that you like. You don't want it so short and narrow it stretches the cotton interlock fabric. Take scraps of interlock and sew a straight seam with the Aurifil. Then experiment going over it with the polyester thread to figure out what zigzag settings works best. Even if you end up reinforcing only some of the seams, it will help.

(Edit: Even not covering the Aurifil thread would help as, even if the Aurifil breaks, the zigzag polyester thread will keep the seam together. Experiment with placement of the zigzag to see if you want to cover the Aurifil thread or not.)

I hadn't thought of quilting over the seam lines, but that would be a possibility too. I really like your original quilting design pattern and was thinking of keeping that pattern but just changing the straight stitch to a very narrow zigzag so the quilting lines have a little "give" to them. You could certainly do a free-motion modified stipple for the quilting. I'm just thinking if you line the top with muslin, that wouldn't be necessary. From a design standpoint, I just like your original quilting pattern better.

If you need to order the polyester thread, redrockthreads.com is a good source. Isacord 40wt polyester would be good. Another good source of polyester thread is metroemb.com ; either Glide or their Metro brands would be fine. Their mini spool has plenty of thread on it and a single spool would be enough for your project. Whatever you get, it should be a 40wt polyester. In terms of color, I would use either a silver or gray as these tend to blend with everything and are in keeping with your fabric colors.

Also, I realized there would be no point in lining the fleece backing. Lining the top will be enough. The backing will be secured to the muslin by means of the quilting. You were able to quilt it before without stretching the backing out of shape, so you will be able to do it again.