butt joint weld?

[parsonsj]

If you are welding sheet metal, the best way to ensure warpage is to leave a gap. The hot tack welds will tend to contract, narrowing the gap. That will be happening all the way around, putting a lot stress into the area. The result is usually unplanned warpage.

In other words, it is better to not leave a gap if the resultant panel is otherwise not firmly held in place by other forces such as braces, crossmembers, or even bends or rolled beads.

jp