I'm presently working on my 4th t-shirt quilt. (I have 4 grand daughters) I never questioned how they were washed and I never had a problem with the interfacing. 40 sounds like an awful lot of t-shirts to me though, although mine were all twin size because they were made from their high school t-shirts and they wanted to take the quilt along with them when they went off to college and the beds in the dorms are twin size. Good luck and have fun.