Warpage

Water over the dam now.

Grind off that excess welding without warping it more. Rough it back in as straight as you can get it using a long straight edge as a guide, being careful not to stretch the metal in the process and create oil canning and warps in other areas.

Once the welds are cleaned up and body panels roughed in, first coat mud over welds should be a water proof filler like Duroglas from USC company. That welding job is no way going to be water proof, need to seal the weld area inside and out so the next steps of bondo won't bubble back out on you over time.

Most classic cars have a fair amount of bondo in them. The trick is finding someone with the skills to get it straight so it looks nice and it is not waving at you when it is finished.

Looks like he used too big of wire in his wire feed, like .035 ths. that is used for welding plate steel together, wrong gas mix too.