Here's another joint that you'll likely encounter on your truck. A plug weld is a technique used to fill in holes or plug-weld two pieces of thin material together. To fill a hole, start with the gun at the 10-degree travel angle and weld around the edge of the hole, making complete circles and spiraling inward until it is filled. | 
This is what the plug weld looks like from the back of the material. A quick hit with the grinder and it'll look as if there was never any hole at all. | 
If you intend to do any chassis welding (thicker metal) or want to weld aluminum components, you're going to need a welder that can handle the increased demands for power and process. The Millermatic 251 seen here is connected to 230V power and offers an optional direct-connect spool gun for MIG welding aluminum. |
A common weld found while fabricating on trucks is the vertical down position seen here on mild steel. Vertical welding, both up and down, can be difficult. Since you are fighting gravity, consider reducing the voltage and amperage 10 to 15 percent from the settings for the same weld in the flat position. | 
Begin at the top and work your way down. This technique helps when welding thin metals because the arc penetrates less due to a faster travel speed. Let gravity be your friend; maintain a 45-degree work angle between the nozzle and the joint. When welding, angle the gun slightly back into the puddle at a 5- to 15-degree angle. | 
For thin metal where burnthrough is a concern, angle the gun slightly up and pull it downward, keeping the wire on the leading edge of the weld puddle. |
The Spoolmatic 30A spool gun has a built-in drive system for hassle-free aluminum welding. Because aluminum wire is softer than mild steel wire, it cannot easily be pushed through a drive system and gun liner without bird nesting. A spool gun eases the delivery of aluminum wire by shortening the distance it needs to travel. | 
The spool gun has three connections seen here: the gas hose, control cable, and weld power connection. Remember to set the welder for reverse polarity (DCEP), which is the same for mild steel. For welding aluminum, use 100 percent argon shielding gas with a 20-40-cfh flow rate. Proper gas coverage is key around the weld puddle so that the arc and argon cleans off the oxide coating on the aluminum, creating good fusion. | 
When welding aluminum, the push technique is the only technique that ensures good gas coverage of the weld puddle and provides better cleaning action. |
MIG welding aluminum progresses at a faster travel rate than steel. Due to aluminum's high thermal conductivity and low melting point, the travel speed will increase as the welding progresses. If the travel speed is not increased, there is a chance of excessive melt-through on thin aluminum parts. | 
When exposed to air, aluminum forms a hard oxide layer that completely covers its surface. The white area seen around the weld is the result of the arc and argon gas cleaning the oxide coating off the aluminum. | 
Always use the push technique when welding aluminum to ensure proper gas coverage, as seen on the left. The weld on the right is an example of the drag technique where there was not enough gas coverage to clean the aluminum. |
This chart shows two types of weld bead patterns, the Stringer Bead and the Weave Bead. The slight motion used in a weave pattern is helpful in covering an area with one pass of the electrode, and when done right, the bead will resemble a stack of dimes. | 
Whether you weld occasionally or you pound wire like crazy, practice your welds until the technique becomes second nature. This diagram shows examples of a good weld and several bad MIG welds. | |