seam sealer underneath floorpan

been searching, haven't found the answer i'm looking for. i just finished welding in new floor pans, now time to seal everything. top side was easy, but what do i do underneath? most of the writeups i've found just say they used undercoating on the bottom side. is there any need to use seam sealer there too? any reason not to? i'll add bedliner to the floor side and plan on undercoating bottom. just didn't know on the seam sealer. thanks

I added to both the top and bottom...not sure if it was necessary but I only drove it for the next couple months then parted it out.

I'd do that method again.

add it to the top and bottom if ya can, then prep it good and undercoat it.