welding technique question

[groho]

Doing some sheetmetal work on an old mustang. Learned allot about heating, warping, blow-thru, etc. I've gotten better, especially my confidence level, but there's one area that I find troublesome; butt welds. Both sides of the sheetmetal has a tendancy to pull up through shrinking and contraction after it cools. My welding is good, good penatration, minimal heat zone, but like clockwork the two pieces will warp just a little immediately adjacent the weld. Often, I can't get behind the pieces to provide support or an opposing weld to anticipate shrinking. I recently set a door gap on a quarter panel with a relief cut, welded, ground, looked great! Then...... Sure, less than a 1/16" of bondo will cover the prob, but how do you prevent it in the beginning?

[steelman05]

If you are using mig, slow down and stagger your weld. instead of doing a long pass, do small tack about 1/4 inch and not allow the metal to get to hot.

[Metarinka]

high localized heat input, skip welding and tacking can help. So can back welding. The other big thing is preheating the whole area so there's no significant difference in temperature.