Butt welding sheet metal is a very important skill to develop for replacing sheet metal in your restoration. Joint preparation and welder setup are the keys to a successful job. In the above video tips are given for the actual welding process as well as some setup information. There are a large variety of welders on the market and in garages and setting one up can be unique to the machine. If you have a 110V welder and you have independent settings for the power and wire speed, then a good starting point is your third highest power setting and 35% wire speed. This is for .025" wire, which is what we use.
From a technique standpoint, you need to practice to get the dwell timing right on how long to make each tack. Too long and you build too much heat and blow through, too short and you don't get good weld penetration at the joint. AS ALWAYS KEEP THE NUMBER OF TACKS TO NO MORE THAN ABOUT 6-8 AND STOP AND COOL THE AREA. In not doing this you will likely cause a good deal of distortion which will require unnecessary body work to fix which is time and money - so why not avoid it?
As you can see we use shielding gas for our welds. We have been asked if flux-core MIG will work and frankly we recommend against it. Some may have success with flux-core, but it's really an inferior way to go with thin sheet metal and weld control. In some cases you will be patching structural areas and there great weld penetration is a necessity, so if you can beg, borrow or steal a gas shield MIG setup we would highly recommend it and your life will be much easier.