As a pipefitter working in metal trades, i have found welding carbon steel with TIG much easier when the copper wash is completely cleaned off the filler material, using a rheostat or foot pedal on fine work is almost a necessity to avoid "butt-holes" someone with some experience with the rig can "walk" the heat away to allow the metal to cool slowly, but a 'stat sure helps.
I have welded all kinds of metals, titanium, aluminum, AL6XN, hastalloy, stainless, carbon, P22, P91. Welded in Nuke plants, powerhouses, and food grade facilities. My best advice, take your time and don't be afraid to file! Make sure all materials are clean, and let the heat be your friend.
