Some weeks ago I started a topic here about Sulky's 12 wt. cotton thread- which I like very much. I mentioned I was using a size 18 sewing machine needle with that heavy thread in buttonhole stitching around fused applique. A few here mentioned whether that size 18 needle was a bit of overkill- and I agreed- but it was all I had unless I went down to a size 14, so I used the 18.
I just started sewing with a newly purchased 30 wt. Sulky cotton thread- buttonhole stitching again- and noticed on the spool's label that it suggests using a 90/100 (size 16) Top Stitch needle. I'd think the size 18 for the 12 wt. thread might be right on after all. :-)
I do love those heavier Sulky threads for this purpose! It really looks nice.... the 12 wt. makes a stitch that looks like a very narrow braid has been applied to the edges of the applique. If you've forgotten here's a pic of the stitching on the leaf in variegated green 12 wt. thread (the flower has the very same stitch in a 50 wt. Sulky cotton thread and looks entirely different)-