This is very interesting to hear from someone who should know what she is talking about. I used the lightweight interfacing and a wide zig zag on a couple charity quilts, because I wanted to see if I had any issues with the long arm going over the interfacing part. I had no problem at all. I don't use Mountain Mist any more either, because I had used it on some of the GKs baby quilts and noticed them getting all lumpy over time. I now only use Warm & Natural or Warm & White.