Jim, thanks for the practice offer, but I do have quite a list going right now...
Welding sheet metal is a different animal than welding up something like thick steel angle in that the sheet metal is not as forgiving to excess heat. For example, if you were to start your weld bead with your mig welder set up properly for the gauge sheet metal you are using, and you were to actually do a weld pass you would see the start of the weld had little to no penetration due to the metal still being cold, as you progress the metal is heating up and you get better flowout on the bead and penetration of the weld, and if you go much further you will blow a hole. To combat this problem, as well as minimize heat buildup and deformation of the metal, most suggest using the tack weld process when using a MIG. I will typically set my machine up for the next highest setting than the metal I am welding, for example, if welding 18 ga, I will use the machine setting for 16 ga. When you weld the first tack, you should now see full weld penetration due to this higher setting (look at the backside, check for this when you first set up the machine). Now if we were doing a weld pass it would surely blow a hole from the excess heat, but because it is a single tack that chance is less likely. If you do tend to blow a hole using this process, before reducing heat, try turning up the wire feed speed a bit. (If you have adequate heat to perform the weld process and the wire feed is not providing enough filler material, somethings going to burn away, thus the blowout) Skip around to minimize heat, and planish the weld "dots" as you go to restore the metal shape (the weld process normally causes a slight shrink at the weld dot, planishing, in moderation, will stretch it back out again. As far as cab corners, if you need to planish here (or other tight spots), then I suggest you make a dolly to suit the application. I have a few dollies that are nothing more than a steel flat bar about 1/2" thick that have been heated and bent into all kinds of odd shapes, then ground down to the final shape.
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