If someone wants to fill small holes and smooth, body filler is the product to use, not JB Weld.
I disagree 100%. Especially where water is likely to be wicked in over time. Body filler is nothing but a weak 2 part resin mixed with talc. Also it isn't uncommon to be left with a relatively porous finished product, especially where it goes through holes. J-B Weld is infinitely workable, is very non-porous when cured and will do a much better job. Besides this is an area with small ledges and lips. That's perfect for working with J-B Weld and if you're careful, you probably won't have all that much sanding to do. If you start MIG welding, it won't be long before you discover the seam where the top joins the pillar was paddled with lead when the truck was built...and you'll get to re-do all that work now that you've melted it.
