Help needed...

You could use seam sealer on top of the inner fender. When I restored my car, I used nothing. I stripped my entire car (including the back side of the fenders) to bare metal, sprayed on two good coats of Epoxy primer, several coats of high build polyester primer (most of which got sanded off) one coat of sealer, three coats of base coat and three coats of clear. Even though a lot of the primer got sanded off, I am I feel very comfortable that rust can never form there.