I have a flux core MIG welder and want to weld some patches into the rear of my Sammie.  Has any body had any problems with burning through the sheet metal?  I welded a piece into the floor board and had some difficulty with it.  I want to make the outside look as nice as possible so that I can paint it this summer.  I turned the wire feed up and the arc volts down but still burnt through.

Are you doing a long beed or are you just doing spots. Which if I rember right is the way to do thin metal.  I the idea it jump from spot to spot to let the metal cool down some and not warp or brun through till you have all of it weilded in.  
George

I started to do a bead and found that did not work at all.  I then tried to do spots and that worked OK.  Still burnt through at times.

 Great white north bodyman to the rescue, just kidding.
flux core is totally wrong for japanese tin(to thin),it can be done but it's very difficult and you gotta clean up the flux before and body work is done, use CO2/argon mix gas (works the best). If you don't have access to gas you can ue panel bond, I know it sounds bad but this stuff gets used more than welding in the bodyshop and is industry approved. Go to an auto paint supply store and ask for brands like Duramix, Proform,Fuser, and I think 3M has a line, also ask for detailed instructions.
 If you don't think this stuff is strong, go look at the roof of a Saturn car (6-8 spot welds the rest is all glue)

I just use .23 wire with tri mix and turn the heat way down. You might try reversing the polarity maybe it would burn cooler? or get some thick copper pieces to hold on the other side while welding.