Another way you might rescue those 12" blocks, rather than trying to add a very narrow sashing around the edge... depending on the design of the block, you cut them in half top to bottom and side to side (dividing the block into 1/4s) and add a 1/2 inch or 1 inch (finished width) sashing forming a plus in the middle of the block and then trim it down to 12 1/2 inches, now forming a 12" finished block. It won't look as much like you are trying to make a block bigger as add the sashing around the edge does. A friend of mine just made a quilt that started with log cabin type blocks that were then cut in 1/4s and black sashing was added to the middle making a + in the center of each block. I haven't seen the finished quilt yet, but it is for her son and he loves it.