Hi Rehd, when I did temporary butt reinforcements with 4oz cloth on both side there was still some weakness while handling, white stress cracks. When there wasn't any problem I ended up sanding through the 4oz when before laying on the exterior glass anyway. I'm hopeless for overbuilding but don't like to have to go back and redo stuff in the midst of building either so my $.02 is to cut 2" wide 6oz on the bias for both sides, when it's all stitched together the exterior gets the 6oz hull cloth on top of the 6oz strip and the interior gets two or three more layers of 6oz. on the joint. I actually like butt block because it's easier than messing with all those layers, and if it's a deck you can't get much stronger than 8mm ply with layer of glass on that.