I agree with a stiff starching. If the fraying is bad on the already sewn blocks I might run a second line of stitching about 1/16" from the first and a tighter stitch. I had to do this with some homespun fabric I used in a quilt because it raveled a lot. I also stay stitched the rest of the homespun blocks before including them in the quilt -- this about 1/8" from the edge.