Everybody has their opinion as to what is best. I would paint the backside with POR15 after welding and maybe spray POR15 in the hard to get spots. Spray might be the wrong word. I have in mind to make a wand out of flexible copper tubeing with the end pinched almost shut or with a nozzle on the end. Use low low air pressure to blow POR15 in the hard to get at seams.

Haven't tried that yet, so it's only a theory.