firewall rust solution

[mentlmidget]

86 c10. I took the factory air conditioning out to install aftermarket air (I could never get the air to blow really cold or in great abundance).
I have a rust problem on the firewall. Specifically as I can, I am talking about the cabs firewall meeting the vented cowl in front of the windshield. Water is going down the vented cowl and sitting where that panel is welded to the cab's firewall.
Any idea on how to fix that problem. It is not in one location. It appears to be along the entire firewall. It isn't too bad right now, but it will be a major problem in a few years I'll bet.
Suggestions please.