Hello Tommy, is this work code work? You had mentioned using a CC power source, you will likely need to qualify a procedure with this if it is to a code.
14-16 GA. weathered/rusted steel can be a bearcat any way you cut it and whether you use a self-shielded or gas-shielded electrode vertical up/down or otherwise will be difficult when dealing with a lot of contamination. I am assuming that you are having both external and internal rust issues along with other contaminants to contend with. Are you able to do any prep work on the joints? ie. grinding, blasting, or anything like that?
You may wish to consider one of the E71T-GS wires as I believe they are a bit more tolerant to contamination issues and can be run either vertical up or down. This wire is not a code wire, however.
Certainly don't envy your job there by the sound of it. ;You'll likely have to do some experimenting til you find something that fits the bill the best. Describe the application/challenge a bit more if you can, include some pictures, myself and others might have a few better suggestions/answers if you can provide a bit more detail. Good luck and best regards, Allan