Another trick I learned about butt welding, is very useful IF you have access to the backside of what you are butt welding. Take a piece of copper water pipe, and beat it into a flat pice of copper. Hold it on the back side of where you are welding....right up against the crack you are welding together. This will keep the weld from blowing thru, as the weld will not stick to the copper. Doing as suggested above (spot weld,cool,grind,cool, fix tiny holes or move on and repeat) is the best way to do butt welds. Good luck with your project. :D