1/4 install snafu

The weather`s been good so for the last few days I`ve been working the right rear 1/4 panel on the Dart. Everything went great , even welding that very thin panel to the thicker factory metal. The problem I ran into was, as I did the grinding on the welds to smooth everything out the panel went concave on me right at the weld seam. I tried to go easy and not create a lot of heat. Has anybody else had an incident like this while installing a 1/4 panel? I could just use body filler to fill it but it would be 1/4 - 3/8 in. thick in some places. I`ll try to post some close-up photos when I get the chance.

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