Got some inside pics? Looks like some plug weld spots from underneath... Who did that welding? Might not need any more.
I wanted my floorpan repairs sealed from underneath too. Didn't trust seam sealer for exterior. Used Black Jack Neoprene Roofing caulk. Stuff sticks to anything. Cures flexible and while I wouldn't expect a fine finish, I have undercoated and painted over it with good results..
