I have done some machine appliqué, though Most I hand appliqué. I was thinking when you use a satin stitch, do you start off by back-tacking to lock your stitch at the beginning (as you near the starting point after satin stitching around, you cover that back stitching anyway) and especially back stitch with a straight stitch at the end to lock? I don't think a satin stitch locks if you only back stitch with the satin stitch. Hope this makes sense and if I am wrong, someone correct me!!