I had an old pieced top that my mil had in the attic that had dried blood on it as well as significant yellowing. The person I had do the quilting soaked the whole thing in BIZ in the bath tub for several days and it came out looking brand spanking new. She warned me before she soaked it that if anything was going to happen to the integrity of the piece she would rather have it happen before it was quilted rather than after. I agreed and it turned out beautifully. I will say that the tops had been finished prior to soaking which is not the case with your piece. I think that I would finish it enough to square it out and then soak and sash it out and quilt after. That should keep it from stretching any on those bias edges when wet. Ann in TN