I used a pattern for woven fabric to make a top out of cotton interlock fabric. To compensate for the stretch, I used one size smaller than I would normally wear. When I tried on the top after sewing the pieces together, it fit perfectly. I loved it and looked forward to wearing it for SSS Day # 18.
But when I top stitched the neck and armhole edges, they stretched all out of shape, and the top was unwearable. To salvage it, I gathered the edges. I was afraid the stitches would pull out, so I stabilized the neck edge by sewing in some flat braided trim. I kind of like the way the neck looks, but the armholes look weird. Still, the top will be fine to wear under a jacket.
So I decided I absolutely must figure out how to keep the knit fabric from stretching when I top stitch the edges. I could have used interfacing on this particular top, since the neck opening was large enough to slip on over my head. But what about when I want the neck edge to stretch -- when I wear it, not when I'm sewing it?
A Web search turned up the following:
(1) decrease the tension on the presser foot;
(2) use spray-on or paint-on water soluble stabilizer
(3) use elastic
I tried decreasing the presser foot tension and still had stretching. I didn't want to use elastic, because part of what I like about this fabric is its softness and light weight. The elastic would make it feel heavy; besides the elastic would look weird on the edges of the armholes. So I went to the fabric store after work and bought some spray-on stabilizer and some more of the black cotton interlock, to try once more.