Seam sealer. This stuff is supposed to stay flexible. Didn't really need to put it on the welds but I did on the underside and this is all under the floor mat so what the heck. There could be pinholes in the weld so this will seal it.
You have shown me many little tricks on body work, got a little one for you that a old body guy showed me.....
After you put the seam sealer on take a brush with gun wash and paint the seam sealer, makes it lay flat and smoooth..

Rob