Get a chunk of 1/4" thick copper to back it up and soak up the heat (maybe a 2" x 4" piece). It works well for filling holes that you've blown through. Also, I had good luck mig welding for 2 or 3 seconds pausing 2 or 3 seconds welding for 2 or 3 seconds, pausing 2 or 3 seconds etc., keeps it from getting too hot and lets the weld solidify.
