seam filler question

Or they would all be rust buckets. The Barracuda had undercoating (dealer applied?) in some places, only primer in others (in black, red, gray, and even pea green colored primer), and some places I can't tell if the parts were even painted. I am fixing all that little by little in my garage. Never done this before so I am lucky to have such a rust free car. The Missouri cars I have.......another story. Yes, if only they could fight rust themselves.