streching panels that arent accesible from behind

[ymhjrhd]

Can anyone give me so info on how I can get a warped panel back to its old flat self after being welded on. I took all the proper precautions and I still had warpage when I welded on a drivers side bed replacement panel. I took my time and made sure that the patch panel fit snugly and tacked every inch then ground down the tacks and welded between, but still the bedside warped on me. I cant access the inside of the bed so am I up a creek? Any help would be appreciated. Sorry about the terrible format of this question.