I am attempting to replace the sheet metal at the rear of the trunk panel by welding in donor panel(the original panel had rusted out around its weld seams due to rear ender early in its life). I am using an Eastwood Mig 135, 75/25 CO2 Argon mix, .23 wire, 20psi of gas and settings of 3 on both heat range and wire speed. I am starting with this piece for practice so that I can properly replace the quarter panels and parts of the floor. Straight line welding is goin well but I am having blow through problems trying to fix holes and mistakes. I am particularly concerned about doing proper butt welds when I get the quarters replaced as I had not planned to overlap any sheet metal on the out side.
I was wondering if any one had any suggestions for welding thin sheet metal or sources of information.

 

i have had good luck doing butt welds by securing a
piece of sheet copper tightly behind the weld area and
welding in intervals of 1/2"or so-the weld will not stick
to the copper and the copper also sinks away heat.
i have used Magnets to hold the copper strip in place,
but very strong magnets seem to effect the weld arc...
Using C25 and .023 wire in my 1983 hobart mig-man.
Fixed up a rusty old dodge pretty nice with this method
and equipment Good luck on the repairs!

 

gotta learn the stick and move dance, have welded up paper thin metal, takes time.

if you got extra id flange your edges, $20 visegrips tool one or $40 air. can "lay" into it better to weld deeper easier so you can take allot off later.

 

Random thoughts:

Turn the heat waaaaaay down. As long as you have discoloration of the metal on the back side, you have enough penetration for the job.
The copper backing is a good idea.
Make sure your weld area is free of paint. i.e. clean metal

 

+2 on the copper heat sync