The other thing to remember is read the directions and use the water temperature they suggest to remove the thread.
On the one and only trapunto project I used it for, I used the wrong temperature to dissolve the thread and it got brittle instead. I had to pick it out. Not fun.
Actually, these days I like using 100% wool batting instead of using the WS thread trapunto technique. As long as there's some dense quilting around the feathers, they really pop out and there's no 'stitching-it-twice' or batting to cut away.
Another use...I've machine basted quilts (after safety pin basting) with a North-South and East-West grid pattern for quilts where most of the quilting is on the diagonal. Then washed it out. Water soluble thread in both needle and bobbin.
Julie