dried sandwich spread sealer?

My Barracuda came from California and was undercoated. It has almost no rust, other than surface stuff, and was not to hard sanding down (by hand with hand tools in my garage). Other than the behind the brace on the inside of the rear valance panel and where the spare tire part of the trunk meets it as well, I've been able to get to almost all of it. But I noticed rust wherever this stuff was applied between the fenders and the engine bay. My worry is, if I put anything else in its place, will it lead to rust once I reassemble the car? Thanks everyone. Great ideas.