I enrolled on an autobody class and carted my tailgate as my first project since it was the easiest to remove. After blasting and welding up the small holes, I cut out a good size area of the bottom roll and cut and bent new metal. When I removed the bad piece I could see that the interior had surface rust. I asked the instructor if I should do something to it. He poked it and said "Nah, looks pretty solid." But I'd just removed a sizeable piece that was swiss cheese.
So what do you think? Pour in a rust inhibitor? Or leave it be. There are drain holes on the bottom anyway...or what seem to be drain holes. I guess I could do it later, but now I can see what happens.