While cars began switching to unibody frames in the 1970s, trucks, which require much more strength, continue to have traditional frames with frame-rails. This is a much heavier metal than what you'll find in modern car chassis. So it's critical that you evaluate the strength requirements of your project. Reattaching a mount for a spare wheel, attaching hydraulics for a dump bed, and repairing a frame-rail itself, each requires a different level of strength. Your welds need to match the strength requirements of your project.
MIG welding is one choice for a lot of automotive projects such as welding tabs onto a frame or doing minor repairs and you can achieve maximum penetration with your MIG setup. If you're reinforcing or repairing actual frame rails, you may need to use a stick welder. It's harder to make your welds look good, but stick welders are generally capable of creating a stronger weld than MIGs.
Welding is a highly technical skill that takes years to master for most people. Trying to work in an awkward position only makes it more difficult. You may need to remove the bed or the cab of your truck to get the appropriate access to do your work. If you don't set up your work properly or have the right work angle, you won't make a good weld.
This is one of the most important tips in welding. Keep your work space and your work material clean. That's a big part of the job. You may clean off scale or rust with a disc grinder or other abrasive. Make sure it is clean of grease or oil as well. While it's possible, especially with a stick welder, to increase the amps so it burns through scale and rust, you don't want to do this. Pick the right amps for a clean work surface. If you leave some carbon, come back with a wire brush to remove it before you start again.
Understand that factory welds and spot welds are often made as economically as possible, providing the least amount of structure necessary. Don't let spot welds and factory robotic welds be a repair guide. You're doing hand welding, so it's not going to look like a factory weld; it will be heavier. You may want to reinforce factory welds while working on your frame.
