What would be the best way to stop rust in a seam. I've discovered a bit of rust on the tailgate of my wifes 2003 Dakota where the outer skin wraps around to the inner panel.(like on a door skin)
I wire wheeled it, coated it with some etching prime with my airbrush, then hit it with some rattle can paint that matched pretty well (silver)
Now just a few weeks later today I noticed a bit of rust stains starting again at the seam. So I guess its coming from the inside out.

There are a few slots that would allow me to pour some sort of rust converter/killer in there just not sure what to use that'll work

Any advice or tips appreciated............thanks