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The problem with tig welding carbon steel square tubing is that is is easy to melt thru and pull oxides into the puddle. When that happens, the puddle gets weird. Pulse settings can help.
One of the biggest struggles for new aluminum TIG welders is learning to feed TIG wire. In my opinion, it's one of the hardest parts of aluminum TIG to get ahold on.
Whether its a vertical weld or laying flat on the bench, the same techniques apply, but getting comfortable can be the main thing. The ABC's of welding are ...Always Be comfortable