Mario, to clarify, I do fit up everything tight, Tig Or Mig. Fit up and lots of tacks as you go so the two planes of the metal are perfect and hold dimension is the most important thing. I said I like to tig everything and hammer and dollie the weld as I go. If you don't the heat and cooling will draw them together and start warping panels. Thats why you need to stretch the metal. But,If you can't tig or just have a Mig It can be done. You want to set the machine as low as you can but still get a good bead and penetration. And I don't mean pull the trigger and take off. On off on off and cool the weld. Trigger in the right hand blower in the left. If you start with a gap and just start tacking away the cooling will just draw the part closer as you move down the piece. Just try to fit it tight and lightly score it with the edge of your cut off wheel after it's tacked up if you need to . If your blowing through it's to hot or your torch angle or approach to the weld is wrong. If it bird turd and poor penetration it's to cold. I found you can get good results with the mig if thats all you have but the welds will be more brittle to hammer and dollie. A good 110 mig and 023 wire (no flux core)is all you need. Those 180's and 210's are just to big for light gauge work, they like alot of heat to work.
The mandrels you want to use are the Matco part #HMANF15.
Hope this helps. Don't know if info is right or wrong but works for me.
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Jason
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