I still think a layer of fine muslin spray basted to the quilt top before layering is your best bet. For those of you who might not be familiar with this quilt, here is a link to a previous thread about it:
Preventing seam separation
Scroll down the thread to see some photos.
This quilt top has a mixture of fabrics, with cotton interlock used as the sashing. The piecing was done with Aurifil 50wt using a straight stitch. Backing is a soft fleece. The problem, now that it is quilted, is that there is a ton of stretch in the fabrics but no stretch in the stitches, so threads break easily when the quilt is manipulated.
This is a very important quilt because it is a memory quilt for a 16-year-old boy. Laura is planning to take out the existing quilting in order to stabilize the quilt for normal use.
Aurifil 50wt is a very weak thread (which I love for normal piecing, but this is not a normal piecing situation). I am thinking it would be best, after taking the quilt apart, to reinforce the piecing seams with a narrow zigzag stitch using a polyester thread. I am thinking Bottom Line for the thread, as it is fine but strong. I would just secure it with a backstitch or two, do a narrow zigzag over the existing seams, and secure again at the end. It doesn't have to be perfect. This would greatly strengthen the existing piecing seams.
I am not sure it would be a good idea to use a fusible on the underside of the top. Non-woven fusibles such as the Pellon gradually disintegrate. They are good for temporary stabilization, but will lose that quality over time. Fusible nylon tricot such as the Stay-Flex would be a better choice, but it would definitely add a little weight and substance to the quilt top. This is not necessarily bad. It will make the quilt slightly heavier and slightly stiffer, but this probably will not be noticeable to anyone but the maker of the quilt.
My biggest question concerning using a fusible on this quilt top are the seams. There are a lot of seams in this top, and they will be relatively bulky because of the fabric mix with interlock. Do any of you have experience using a fusible on top of a seamed quilt top? I am thinking it is going to be difficult to get the fabric to fuse uniformly. Different fabrics may respond to the fusible and heat differently. The seams will make lumps in the fusible, so there will likely be gaps between the seams and the top's fabrics. I have only limited experience with fusing, and it has always been on flat fabrics with no seams. It seems to me there is a possibility of disaster in this case.
I still think the best option is to line the top with a cheap, fine muslin -- the type widely available in stores, including Walmart. It can be purchased in a width bigger than the quilt top so it will not need to be seamed. It can be prewashed and dried a couple of times to ensure that it will not shrink inside the quilt top. And it can be spray basted to the quilt top. Spray basting is a much more forgiving endeavour than fusing with heat, as the fabric can be lifted and re-arranged until very smooth before heat setting (or allowing to dry naturally). The muslin will not gradually disintegrate inside the quilt the way a non-woven fusible will, and it will not add as much weight (or stiffen the drape) the way a fusible tricot would.
I am wondering if it would be a good idea to also line the fleece backing with muslin. This would help relieve stress on the quilting stitches.
For quilting, I would use a polyester thread again because it is stronger than cotton. Bottom Line (60 wt) would probably be too fine. I would consider Glide or an equivalent 40wt polyester thread. The quilting could actually be done with a zigzag stitch too, just to add some "give" to the quilting. It could be a narrow zigzag, or a wider decorative zigzag.
Sorry to go on so long, but I would like to see Laura get some really solid advice for this quilt. She has put a lot of work into it and deserves to get the result she is looking for -- which is a durable memory quilt for the family.