BELOW IS SOME HANDY INFORMATION FOR MIG BEGGINERS

You can use CO2 as a shielding gas for Mig welding mild steel but not for stainless steel due to carbon contamination from carbon dioxide (carbon monoxide is created from the heat in the weld pool).  Using CO2 as a shielding gas converts MIG welding (metal inert gas) to MAG welding (metal active gas).  Active, being the absorption of carbon into weld pool from the carbon monoxide expelled from the heated shielding gas.

Ready source's of bottled CO2 are pub gassing bottles (cellar gas, tavern gas, sureflow).  Do not use bottles with nitrogen in them (grey bottles for cream flow beers, Guinness, sureflow mixes).  Pub bottles weight 17.25kg when full, 11kg when empty, dimensions are 94cm high x 14cm Ø diameter and they are black in colour (any black bottles marked CO2 with a white stripe down them are for liquid with-drawel and thus no use for welding).  £20 is about the going rate for a full pub bottle at the time of writing.  Another source of CO2 are 4.7kg Fire Extinguishers.  Most fire extinguisher suppliers will refill bottles.  Some pubs now use full size CO2 bottles!  The size of industrial W-size oxygen, argon/argoshield welding bottles (230bar 85kg gross 146cm high  x 23cm Ø diameter).

CO2 bottles show about 50bar (750psi) at 15°C when full and have a 5/8 bsp male threaded fitting.  Standard argon bottles have 5/8bsp female fitting designed for a male bullnose fitting.  So an adapter or dedicated CO2 regulator is needed (adapter/regulators available from http://weldability.com/ ).  Machine Mart sell the cheapest industrial type CO2 regulators. 

CO2 when compressed turns into liquid-ish fluid (thus low bottle pressure, propane/butane & LPG acts very much the same).  When contents of the bottle turns from liquid to a gas it lowers in temperature (due to process of gas transfer from a liquid state) thus you may experience some weld splatter as the cold shielding gas hits the red hot weld pool.  You can overcome this by fitting a CO2 heater directly threaded to the bottle before the regulator. I use a Murex Sirocco heater but a cheaper version from http://weldability.com/  is available.  At very high flow rates (building ships/bridges)  CO2 will have to be heated as the low temperatures cause vapor locks and damage to regulators/gas lines.  Splatter is also caused by the Arc voltage increasing in CO2, a different type of Mig welding, takes place above <200A in CO2, spray transfer, this has some good side effects i.e. improving weld penetration in really thick plate.  If you can't live with a bit of splatter on thin plate when using 0.6mm wire, you will need Argoshield Lite from BOC, Argoshield Lite also leaves the weld bead with a flatter profile than CO2, which is handy when welding repair patches on cars, saving time on post weld grinding.

Most low amperage (>150A) Mig/Mag welding will need shielding gas supplied to torch at a pressure of1.5-2.0bar (20-30psi) and a volume of 5-7 litres a minute (welding outside in windy conditions will require more gas, about 10 litres a minute and/or physically shielding your welding with a jury rigged windshield, plywood or sheetmetal).  A Bobbin Type Flowmeter to set the flow rate between regulator and mig torch is a must for economical usage of gas.  It will pay for itself very quickly.

More expensive MIG welders have spot welding timers, we have two MIG welding sets, one of ours is fitted with this function, I have never used it (total waste of money).  If you do use the timer, you will need a special nozzle shroud with slots in the side of it.  Welders with reverse polarity function are handy for using different types of wire i.e. gasless (flux cored).  Solid wire sizes 0.6mm to 0.8mm are needed for all auto repair work.  Spools come in 0.5kg 5kg 15kg sizes,  5-15kg size far cheaper if you are going to do any amount of welding.

Alumininium, stainless steel and mig brazing must be done in pure argon