Patch panel welding - body guys

A used bed will rot out. The foam they put in there holds water. Unfortunately there is no way to get in there and remove it without removing the whole panel.
I agree with oldman, I always use a skim of short strand first and sometimes follow up with USC all metal or evercoat metal-2-metal. This will waterproof the seam. If you can get to the back side a bead of seam sealer is advised.