This is an example of MIG welding thin metal as would be done for local body repairs.


(click for larger image:)


Welding body repairs can be tricky but it is not as hard as it seems. There are 2  problems with welding thin metal. First it is easy to burn through. Second heat saturation warps and distorts body panels. It takes very little heat to change the shape making welding on flat body panels difficult. 


My choice of welder for this is the MIG using a "stitch" process. This allows a short quick weld without heat soaking the panel. The process can proceed as slow as you like and is quite easy.  The following is an example of the process using a piece of scrap metal.


The choice of welder is a basic MIG using Argon with 25% Co2 gas. Nothing special here!

I used .024 wire and heat setting comparable to heavy gauge metal!

First I cut a piece of scrap to a rectangle, then roll it into a tube using the brake/roller/sheer from Harbor freight.  Using a flange tool (under $50 from Harbor Freight) I make a lip for the metal to over lap. The lip is very helpful as it provides a double layer of metal for welding while keeping the panel flush.
Clamp the metal together with a small gap. The small gap is useful for the spot weld to lay in.   Metal MUST be clean. No paint, no oil, no rust! 

Holt the tip about 1/4-3/8" from the metal. Hold the trigger for about 1/2 second. It should be a 1/2 second buz. Space the welds out to hold the metal.  

Remove the clamps. Add spot welds in between. Take your time. Let the heat dissipate in between welds. You should be able to touch the panel at all times with your bare hands except where the spot weld is. If the panel gets hot to the touch you are going to fast.
Slowly fill in the gaps with spot welds. Slowly. Don't let the heat build up.  Sand the top of the welds down but AGAIN, don't let the sanding generate enough heat to warp the panel. Take your time. Sand a little and wait.  The inside shows good penetration and some color discoloration. 
  That wasn't hard was it!