If you are having any difficulty with butt welds, try a copper backing bar. Don't try any other metal, except silver, as bad things of various different kinds will happen and ruin the weld, or you, or both.
Some use a flat bar, and when the weld penetrates properly, the back side will be left flat, almost. Others use a grooved bar, and allow a small bead to form on the back, which is good. For serious production line welding a water-cooled backing bar is used. If you use say 6 inches of 6x30mm copper, or better, 8x30, and do a 6 inch run at a time, just dunk the hot bar in a bucket of cold water and dry it off after each run.
Molten steel does not stick to copper. The high thermal conductivity of copper (or silver, if you dare to sacrifice the family silver!) conducts most of the heat away from the joint, allowing plenty of heat to be applied to get good penetration without burnthrough or having it all collapse in a heap. It eliminates the need for the "thin metal technique" of basically laying down multiple separate blobs instead of a continuous bead. Because the heat is localised right on the joint line, so is the thermal expansion and subsequent contraction. You may still have to hammer and dolly the joint line after welding to stretch it as much as it shrinks on cooling, but it will be a lot less work than the usual case.
