I've used lightweight woven interfacing and I've used Easy Knit interfacing (both are iron-on) for the t-shirt quilts I've made. Both have worked fine for me, and the quilts have always turned out well...no interfacing lines and no bubbling. I have several more such quilts I need to move on...for my SIL! He'll end up with...two, for sure!...Grateful Dead t-shirt quilts, and another one with various artists on it. My dilemma is picking the right fabric to showcase the artistry of the shirts!