Truckguy, don't ever bondo holes, the bondo will pop out. Use a MIG welder and put everything down low and only weld a inch to two inches long in one spot and move around. Like Matt said, welding in one spot for a good amount of time will warp the hell out of metal, same with grinding, be careful on that too. Take all old bondo off, sand the whole truck to bare metal if possible and get ALL rust and black spots in the metal... Black spots are bad, get rid of em. Spraytech here can mention a good gun to you and I know what it would be.... The new 3M gun. Hope this helps man.