Oh my gosh, here is a tutorial on exactly what I was trying to tell you. AND it is with photos, so easy to follow and it shows white as the 'self-sashing'
http://quiltville.com/stringquiltingprimer.shtml
Bonnie shows that she marks the foundation and she uses the foundation as the 'sashing' strip instead of sewing a separate fabric down the center.
hope this helps
warm quilt hugs, sue in CA