recently I was quilting a wall quilt and had a lot of problem with the back having all the stitching pulling out. I tried everything,new needle, check tension and eventually found I was using 2 different types of cotton-1-Rasant the other Threadart. Once I put Rasant in the bobbin too the problem was solved. But like the others I would suggest a small ladder stitch for the repairs, the stitches will look like the normal machine stitching. Good Luck.