Welding 1/8" to 20 Gauge HELP!

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I need to weld a 6" X 6" plate of 1/8" to a piece of 20 Gauge in a car.The 20 Gauge is vertical in position.I tacked in the 1/8" plate using FCAW with a temp for the 1/8" and turned down the wire speed and put the arc into the 1/8" and tried to butter it into the 20 gauge but had a couple of burn throughs just tacking.From what I have read on this board when welding thick and thin metals together you should use the temp setting for the thick metal so you get a good burn in.  But it seems like I am going to burn through the 20 gauge with this setting.Would it be better to use the MIG type (gas) welding and use the temp setting for the 1/8" or just turn down the temp a bit?  I am going to be running two vertical welds (I would think to weld downward in this case) and top and bottom weld (remember this is being welding on a vertical surface).Should I also run small stitch welds instead of one lond weld on the sides?Thank in advance!Mike
Reply:You want to be running pretty cold.  And since it's 20 gauge, I would just run downhill.  MIG would be the way to go, I would think.  I just welded on a horse trailer with a Lincoln sp 185.  I had the settings on c and 3'ish.. I'll have check the machine to see what the actual voltage and wire speeds are.  But the gas was 75/25.  Also, try to direct most of the heat twards the thicker metal so you won't have a much of a problem burning though the thinner metal.Common sense in an uncommon degree is what the world calls wisdom.
Reply:When you say have the settings pretty cold you mean just hot enough to burn into the 1/8" plate?Also if I run striaght down and don't try to butter in the weld will it melt enough to go into the 20 gauge?I also have a bottom weld (like an overhead) and a top weld.  Should I just keep all the heat on the 1/8"?Thanks!Mike
Reply:I am not sure if this is acceptable but why not tack it then do a pass on the 20G to build it up a bit then turn up the heat and get the needed penetration into the 1/8th? How much stress will the welds see?
Reply:I was thinking about making small tacks all around the edges and then do some 1" - 2" stich welds around it until I have it welded solid.Has anyone does this before?Thanks,Mike
Reply:I think stitching will defiantly be necessary to avoid warping the 20G.
Reply:Ok, I will make tacks all the way around it and then run stringer welds.  I think the tacks should help with the burn through.Thanks again everyone!
Reply:Using MIG instead of the flux core should be less likely to burn through. Keep the stringers short and let the metal cool between welds or the plate will warp. Stagger the welds around the sheet until the  edges are filled solid.
Reply:I went ahead and tacked welded around the whole plate today to where the tacks almost make a complete weld.Now I am going to turn up the heat on the mig for 1/8" steel and run stringers on the 1/8" and the tack welds to bond the plate to the welds.Thanks for all your help!