me again, 3M makes a "welt" that is perfect for the purpose of gasketing between the inner and outer fenders. It is sticky and very pliable, it comes in a cardboard box. Two strips, side by side, along the length of the seam are recommended.

I don't know what's worse rust or globs of badly applied undercoating everywhere. I think that the degree to which undercoating helps is questionable, in any case. It probably has a lot to do with how it was applied and how good the substrate was prepared to begin with. Just like any paint job, the painting is the easy part.