Hammer??? Never had that problem and I have made a number of Stack 'n Whack with Bethany Reynolds book on Stack n Whack. When sewing three 45 degree pieces together, you stop short of the end of the piece that goes to the center... By not sewing the on fourth inch seam, it allows the ends to twist on the inside of the piece. The seam connecting the two rows catch all the seams together, so no open seam. I also use Eleanor Burns idea from four pieces together, to let them 'twist' on the back side. Again, its ripping back about two or three stitches on the back side so they twist. I have heard that pressing the seam open, weakens the seam, but have not experienced or seen any where that happened. Yes, even Jenny Beyer supplies offered for sale includes a 'hammer' to use on stubborn seams, so many quilters do use them. I will try to post a picture of multiple seams coming together to illustrate what I am saying. Sometimes a picture is worth more than a 1000 words. Works for me.