On my 83 long bed 4WD deluxe there is a horizontal seam on both sides of the bed where 2 panels are welded together. Even here in dry, unhumid Arizona rust comes back through on this inside jont after being painted. There is no rust anywhere else on the truck. Your expertise please. Thank you. Scott

 

There is no way to stop it completely from everything I've read. Try fringing it down clean both sides and por15 both sides then seal prime and paint. That's what I'd do and will be eventually

 

Low tech, 10-1 water-vinegar on clean bare metal followed by por-15 (or equivalent).

High tech, chemical bath stripping followed by a zinc bath.

Hard core, cut out the spot welds separate and treat the panels then reattach with studs and nuts.

 

May I also suggest trying Permatex Rust Converter to treat the metal and any residual rust after cleaning the metal as well as possible. I applied some of his on exposed metal from wind rash on my truck in 2013 and did not finish with primer nor paint. Yet, rust had not progressed nor come back. This is what I plan to "inject" into the inaccessible recesses of my gutter.

To seal / fill gaps I already have 3M Dynatron 550 seam sealer.

 

it will never stop .. in some spots there are 3 layers , and on a mount 4 layers of steel .
ONLY way to stop it , is to completely dismantle the bed , and rebuild it using new steel , and then , it's just going to rust eventually anyway !!