Rust can start at the back of the seam where the metal has less protection - only primer (maybe) and, if water has leaked through into the {wrong} insulation {for the task}, you could have larger rust issues develop unless your dry out the fibreglass batt.

I injected a rust converter/inhibitor into my seams before sanding/priming/resealing/painting.

On 06-Dec-2014, at 19:39, Andrew R. wrote:

> I've found a water/rain leak in a seam on my van.  Cracked OEM sealant
> letting in water but don't see any signs of rust. I just want to reseal the
> seam with minimal painting or rework.  Is this possible or should I be
> ready to grind, sand, seal, prime, AND paint?
> 
> Thanks,
> Andrew
> '87 GL