MIG Brazing
I tried out MIG brazing and its better than MIG welding. MIG brazing is a new welding technology that's come out of Europe the last couple of years. New cars are using more and more galvanized metal to protect against rust. Galvanized metal has a coating of zinc on it (that's the galvanization). If you use a MIG welder to weld galvanized metal together, it burns a wide strip of zinc off of the surface which results in rework and/or potential rust problems down the road.
Advantages
The advantages for MIG brazing is that it is about 500 degC cooler than MIG welding which means that it burns off less zinc if any. For Joe bodyman like me or you, it translates into less chance of burn through and warpage from heat on sheetmetal. It's basically the equivalent of soldering with a copper wire. It's great for bodypanels but not recommended for structural parts.
Settings
I changed my welding gas to 100% argon and picked up a couple of 2 lb rolls of 0.025" dia silicon bronze wire online from AWISCO N.Y. Corp for $20 US. I had a hell of a time locating a 2 lb roll a couple of months ago but it seems that a lot more welding suppliers are carrying it now.
The standard size seems to be 0.035" dia. which is NOT the wire to use! The thicker wire runs hotter and gives no advantage over regular steel mig wire - same problems: warpage, burn through, etc.. As a matter of fact, I had a harder time trying to mig braze with the thicker wire than regular steel wire.
I use the lowest voltage setting on my welder and a little faster wirefeed then I usually use for normal MIG welding. I did a test lap joint, ground down the brazing so that it was flush and it is strong! I'm very excited about it, mainly cause its a lot easier than MIG welding. The weld bead does not spark when grinding and the color of the dust is copper color. When you start to grind steel, you see sparks. By the way, I used my normal welding helmet lens and could see very well!
Lap and Butt Joints
I had purchased a panel crimper that attaches to my air hammer. It uses the air hammer to form a step for a patch panel to lie flat. It's noisy and works after a little bit of hammer and dolly work. There is a small indentation in the metal where the edge of the panel crimper rests on the sheet metal. It puts a mild crease in the panel that needs to be straightened out.
The same rules apply as for mig welding, you must braze in 1 inch increments 6 inches apart to prevent warpage from the heat.
You can also butt weld with mig brazing but lap joints are the preferred method.
Conclusion
When welding or brazing an old car, the sheet metal thickness will depend on how much rust there is. I was getting quite frustrated brazing a small patch panel just below the rear taillight as the braze wouldn't hold so I switched to MIG welding. Well, what a difference! The welding burnt through the metal immediately and instead of finish welding a patch panel, I had to fill a bigger hole. I was trying to patch to a very thin rusted section. The brazing almost worked but welding just blew right through! It is possible but much harder to burn through with MIG brazing.
MIG brazing works best with clean lap joints, butting pieces together works but you have to doublecheck that there is a clean joint. The bead left by MIG brazing is not like a nice weld bead. It puddles and balls which easily grinds down. There's no spark when grinding a brazed bead - its copper based. Make no mistake about it, even though it is 500 degC cooler, your panels will warp if you try to run a continous bead (experience talking here!). You need to take the standard precautions to prevent warpage - shorts beads, 6" apart.