Some flex is normal but your roof may be "oil canning" from being stretched due to dents or from the sandblasting which is why I don`t recommend blasting large areas.I good body man can shrink the area back up by heating a small spot and using a hammer and dolly to push metal together thereby tightening the panel,not a big deal.I would first weld it on the car and reattach it to the structure and then recheck it for any bad spots.
