With the experienced help of Vic Carnevale, I was able to identify and remedy my stitching knot tying problem. It turns out I was using a tie cord that was not impregnated with enough wax, making the knot hard to tighten without loosening. Vic brought over some of his tie cord from a project he just finished, and wow, what a difference. By comparison, the knots now almost finsh themselves. We cut away the tying that I had already done to make sure this was all done correctly. I was able to redo about half of the stitches this session, and will pick up the rest next time.
