[jonzcustomshop]

another thing that will help with the welding, is to get the paint/surface rust off of the seam that you are welding.
I use a combo of 3 things for this. (together , or seperatley)
paint stripper,
wire wheel (for the 4.5 inch grinder)
36 grit flapper disc (for 4.5 inch grinder)
be careful with the 36 grit, if you dig in too deep you will sand metal away too, but a light touch will burn the paint, and rust right off.

another reason for blowing holes besides what tony covered, is if you are still in the rusty metal. If it is still rusty, the heat will boil the moisture out of it , and what is left will blow through.

I started with what I thought was a 1x4 rust hole on my fender patch, by the time I got to good metal the patch was about 5x12.