Welding gas cylinder, what size? Options

[ipozestu]

I'm about to put a '75 in the air that's been sitting in my garage for a year. This will be my first crack at rust repair. The Hell hole and lower long area is where the most attention is needed.
I'm new to welding, I've picked up so much information here I feel pretty good about starting this.
I picked up a Lincoln 3200HD off craigslist for $200. I've got a regulator, now I need a cylinder. What size should I get? I was looking at a 20lb bottle available on craigslist. It was designated as CO2. Is that the right bottle? Can it be filled with mix gas? Should I get something bigger?
Tips. tricks, comments, suggestions?
Thanks

[Todd Enlund]

Different categories of gas have different threads. CO2 and CO2/Argon both use the same threads, so there should be no problem. You cannot, for example, fill an O2 bottle with CO2. If the bottle is more than 5 years old, you will need to have it tested before anyone will fill it. The easy answer to that is to simply swap it for a filled bottle, and the gas supplier will take care of the testing before they swap it to the next customer. I've swapped several different gas bottles, and nobody has ever checked where I got the bottle.

Don't waste money on a shiny brand new bottle. As soon as you swap it, it is gone. You can have your bottle filled, but you'll really appreciate the convenience of swapping.

The Blue Rhino propane cylinders have a proprietary valve that can only be refilled by Blue Rhino. That's why they are so expensive - they already have your pants around your ankles.