What you're talking about is seam sealer. There are about a million different kinds, for all different purposes. Any kind of brushable, flexible seam sealer will work. You can spend as much or as little as you like. I went with an $18 quart of brushable stuff made by Evercoat that I picked up at Carlisle. The best stuff that I have used is made by 3M, and it comes in a tube for your caulking gun. It's expensive though, around 40 bucks a tube. You've got lots of options here, but the important thing is that you use SOMETHING to seal the seams, otherwise moisture will get between them and start to rust, especially if you did'nt use weld thru primer.