GMAW (MIG/flux core)is the most common process for sheet metal repair and fabrication used today. It's low cost, speed of use and relatively low learning curve is the reason why. Specialty wires are available for use with the process. ESAB EZ grind is one and most any dash 2 wire (ER70S-2) wire in a small diameter (.023/.030) used with C25 (75% argon/25% CO2) are the better choices. Straight CO2 gas is reactive and aids penetration on thicker materials and will increase the likelyhood of opening up holes in the less skilled welder. Dash 6 (ER70S-6) wire can be used where grinding is not required. These and other wires are quite hard and grinding can generate enough heat to warp panels and the surrounding sheet metal will wear away faster. Just remember that dash 2 wires are considered non-structural and you will need to change it out before working on frames and such. FCAW with or without gas (depending on wire requirements) can also be used with good success. Keep your stickout short and use a low push angle and spot and/or stitch weld to control panel heat. Do not cool welds with water or shop air as this will also harden welds and warp panels. Heat control putty is also available. The new GMAW machines with soft start and pulsing are great machines for sheet metal work. GTAW (TIG) is also great for sheet metal through thick section. Much higher learning curve and skill level. Slow production speed. Originally developed as a replacement for oxyfuel. Has the advantage of being able to weld most metals and offers superior control of heat and awesome flexability. Must higher cost for equipment and labor. Miller and Lincoln offer lots of process info on thier sites. http://www.millerwelds.com/education/library.html
http://www.lincolnelectric.com/knowledge/

Jim

Good luck with your project!