Nicole----
Seem's like rust ? :>)
It is !!
All kidding aside, there is one way to get this job done so it lasts, if
in fact the rust is terminal.
Tape off the seam in question with a couple of layers of good quality
(3M) masking tape.
Take a razor knife and cut out the caulking, sand or sand blast ( I use
a concentrated tip, hand held sandblaster) and get as much of the rust
out of the seam.
Remove the tape, ( because more than likely you will have torn it up in
the rust removal process) re-mask, and spray a good coat of zinc
chromate primer, scuff this after it dries with 320 paper, and prime
again with a filling primer.
Scuff this down with 320--400 paper, and apply either a poly seam
sealer, or an epoxy seam sealer.
Let dry over night, lightly scuff the sealer with 400 paper, clean the
seam good, and apply the paint.

It's a time consuming job, but you will be rewarded with a longevity
factor.

You also could just touch up the rusted area with matching paint, but
the results will be short term.

You could also try a product made by Penzoil---Marine corrosion
protectant.
Comes in a spray can.
Apply it to the seam, rub on the affected area with a rag, and it will
remove some of the rust stain , and because it's a cosmoline type
product it will give you some sort of short term rust protection.

Good Luck,

            ______________
           |[  ] [  ] [ ]\
           |    |    |    |
     ~~~ ~||-(())----(())-|

Terry--
74 Campmobile- (Clementine)
85 GL- (Delilah)
86 BMW 325 ES- (Eva)