Two things to remember:
#1: As BLUEPEN said: You don't weld sheet metal with a bead. Use spot welds - jumping from side to side until all the gaps are filled, never heating any one area too much. If you use a bead, you WILL warp the piece.
#2: Make sure there is a gap between your original piece and your patch piece. This is the area you 'fill' with weld. If you try to weld a patch piece in that touchs the original piece, with no gaps, the weld WILL fail.
