I bought my Used Lincoln SP200 6 years ago from a pro welder that was retiring and he gave me the large CO 2 bottle that is all I have been using, but We do a lot of heavy metal welding. I've read the the argon CO 2 mix is better for thinner gage metals, and have been thinking about buying another bottle. I have a large Argon bottle for my Tig welder and have heard that some guys Y up two bottles and Mix as they go. For tig welding heavy Aluminum Hydrogen and helium. is recommended to be added in the argon mix. Welding tips and tricks , com has a lot of welding info posted and Jody WILL answer your questions if you don't find what you need by searching his Free site or Utube videos. I did most of the weldin on the 29 Nash roadster shown when I was taking a streed rod Fabrication class a few years ago at UVU and they used Argon co2 mix.