I had the same problem when I tried to use a whip stitch on hexies. Ended up simply piecing the hexies together, same as one would hand piece any block pattern -- using a running stitch. Jinny Beyer hand pieces this way, if you have access to her book and/or video demonstrating what she does. Her running stitch is very fast because basically she is manipulating the fabric onto the needle (rather than moving the needle in-and-out of the fabric). I never got as fast as she is, but it went a lot faster for me than whip stitching did -- plus, my stitches didn't show afterwards. A la Jinny Beyer, I do a small backstitch every couple of inches when doing a running stitch to piece.