Well...I've been using butt joints for a long time. I rarely have trouble, but when I do I simply do what is needed to fix it. Usually minor patching, probing, feathering, etc. I would not hesitate to redo a complete joint...if I had to. I believe one of the reasons I rarely have trouble is the method I use. First, I butt joint the full sheets and insure that they are straight and level at the joint. I mostly use 7 oz tape with a selvedged edge, but as others have said cutting your own will give you a smoother, easier to feather joint. I lay the tape down and saturate it with a brush, taking care not to float the fiberglass. I lay saran wrap on the joint, cover with a board, weight, and let it cure overnight. Then I do the other side. I set the completed butt jointed sheet on sawhorses, cut what panels I need, stitch, fillet, THEN scrape and feather the joints in preparation for fiberglassing the hull. One thing to be careful of is that when one puts weight on the uncured joint it may cause the plywood to bow at other places. One must apply weight where needed to keep the ENTIRE sheet straight while the joint cures. This will help alleviate the flattening at the joint when forming a curve. I've never had trouble with the strength of a butt joint and in any tests I've made the plywood broke before the joint.