personally I would use a brush on seam sealer over the welded areas then put a bit of paint on the top once its dry. I would do the same underneath but just dab some underseal onto it, this is probably the best way to go. if you find there are holes i would catch as many as you can then skim abit of filler over the top to fill any ones that are left. the seam sealer will protect it.
hope this helps