Yes Argoshield (BOC) and Cougar 5 (Air products) are required for any decent welding. My understanding is that these gases are a blend that includes Argon, CO2 and strangely, Oxygen (strange because the Argon and CO2 are there to stop the weld meeting Oxygen. I can only think that a controlled amount must help the weldpool to flow).
I haven't used pure CO2 for fifteen years +, as its another thing to work round as it tends to give a slightly spattery, hard weld. Not very nice.
It always seems to me that anything to do with DIY welding really does the consumer no favours: CO2 which is a real budget gas, nasty DIY MIG welders, poor panel tools that don't cut/punch/bend, high prices for everything. I suspect that if most people were able to do their first welds with a good quality, professional MIG welder, they'd be amazed how easy it can be.
As a guide, I would always buy an old 'proper' welder, rather than a new DIY one: I bought my Boramig (Cebora) 17 years ago, 2nd hand for £350 (lot of money for me then). It's still going strong after a lot of work, and is probably still worth a couple of hundred quid as well. There's loads out there: generally if it's got a 'Euro' torch fitting (torch cable is quick-release from the machine), its probably a 'proper' welder. (These are the machines that are currently £650 - £900 ish new).
Sorry, don't know where that all came from, was only going to say CO2 bad, Argoshield / Cougar5 good

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Anyway, back to the Bewildebeestmobile: I agree with Steve, that the hardest welding is the outer, cosmetic stuff. I think the challange with the structural stuff is more to do with working out the method of getting the car stripped right down, supported, cut up, repaired, and back together with everything fitting right, and pointing the right way.
The inner and outer panelwork tends to present different sorts of problems: the inner is often all about producing 3D panels with lots of shape, and/or curved flanges. The outer is all about very tight fits and distortion free butt welding. As we've said before though: if there's no huge rush, then it all presents opportunities for problem solving. Although our cars were produced from pressings, that's not to say that an awful lot can't be formed using a few hand tools: including heavily compound curved panesl if needed.
Looking foward to the next instalment

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