MIG Welding sheet continuous beads or lots of tacks?

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I see a lot of pictures of people welding sheet metal (such as fenders) with MIG, and the process usually shows the fender tacked in place and then the next picture is a beautiful clean fender all pretty. I never see what happened in between. My question is, did their welds look as ugly as mine do? I'm welding really thin metal - sheet metal for home ductwork. In my efforts to weld this sheet metal I've made consecutive tacks, alternating the spacing so as not to warp the metal. As my beads get closer together, it really isn't that pretty.If I grind it down, I'll usually find little pinholes in between the beads. It might take two or three tack-and-grinds to get it smooth, but it feels like I cheated.  The series of pools together don't look like the pretty TIG beads, but more like little tiny dookie piles all stuck together. It's irritating.If I try to run a smooth MIG line like I would with thicker metal, it never works. I usually just burn through, even on the lowest current with a good feed rate and steady hand. I was able to successfully run continuous MIG beads on mild steel exhaust tubing (relatively thin stuff from Autozone) but I can't do this on thin sheet. Is it even possible to run continuous MIG beads on thin sheet? I've noticed some improvement when I put something behind the sheet, but even then it is hard to make the beads blend well together. So, can anyone recommend useful techniques for making decent MIG welds on thin sheet? Thanks ahead of time. Perhaps I can post a few pics when I get a chance to help with what I'm talking about.Hobart Handler 140 MIGHypertherm Max 43 Plasma CutterA Bernzomatic Torch, and some solder.
Reply:I have a few questions.... is the material galvanized?What's the machine, settings, gas type, wire size....... and what kind of a joint are you using?_________________Chris
Reply:What are you calling really thin. 28-30 ga ,24 ga ,22-20ga, 16ga. 20 ga should be too hard with a good machine and .023 wire. If you're talking about heating and a/c duct work in your home that not going to be easy. I've used a few cheap 120 volt machines that just had a high low switch for a voltage adjustment. For 20 ga high was a little too high and low was too low. There was one sweet spot that 20 ga would weld ok, however that spot was just that a spot not a range that would work.
Reply:If you can, put a heat sink in back of it. Usually for miggin' sheet metal, I would do multiple tacks, and backfill. It works for sheetmetal- it's the one place where spot welding with mig is the norm. Don't bother trying to grind off much of the galvanizing. You will just burn thru anyway. After spotting it in place, you can use a flex pad grind wheel to smooth it down.
Reply:A series of spot welds is standard procedure for thin sheet metal, but you still want many other things going your direction.  You didn't really give us many details about your machine, process, or material, but uncoated steel, GMAW with C-25 gas, and .023" wire is your best bet.  Using a heat sink is also a great idea; a flattened copper pipe con sometimes make all the difference.ESAB EasyGrind wire is also an excellent choice for autobody work.
Reply:It is galvanized sheet about 0.030" or so which puts it in the 24 gauge area. Using a Hobart Handler 140 MIG on the "1" or "2" setting. Welding solid wire with 100% Ar. I am using this sheet as the bottom tub of a sandblasting cabinet - welding it to 1/8" thick flat bar stock. I've pre-ground the steel sheet prior to welding to remove much of the galvanized layer. It is really difficult not to get spattered welds with holes, and tacks don't always hold. I'm just blowing through the sheet. In some areas I get a good bead. Trying to run long seams is darn near impossible.Is this sheet just too thin? I'm getting kind of frustrated and thinking about starting over.Hobart Handler 140 MIGHypertherm Max 43 Plasma CutterA Bernzomatic Torch, and some solder.
Reply:I'd spot weld or plug weld that and then use seam sealer. My previous post should have read that 20 ga shouldn't be too hard... but for some reason I can't edit it.
Reply:Originally Posted by tresi... for some reason I can't edit it.
Reply:Originally Posted by tresiI'd spot weld or plug weld that and then use seam sealer. My previous post should have read that 20 ga shouldn't be too hard... but for some reason I can't edit it.
Reply:If you rip it off and go thicker don't get any more galvanized. Get some 20 ga paint grip sheet metal and things will go much better.
Reply:Originally Posted by DHillWelding solid wire with 100% Ar.
Reply:Probably a combination; thin sheet, galv, trying to run bead on thin sheet, 100% argon...you get the picture. If you aren't dead set on running bead (which almost always is not necessary) then tack and seal, begin your bead on the bar stock and drift to the sheet and stop immediately. You could try folding a seam along the edge of the sheet to double the thickness, clamp tightly to the bar stock and give it a go. If I'm not too rusty, 100% argon concetrates the arc heat whic is probably contributing to burnback, generally 75/25 mix is standard, anyone jump in here? And good luck!If you don't have the time to do it right, then you definitely don't have the time to do it over.
Reply:I missed the gas, though I did recommend C-25 earlier.  That's definitely a problem.  You need C-25 argon/CO2 mix to get balanced penetration on this material.