Technical Rust stains on paint

Lousy photo but I was buffing out my pickup bed sides today. This one I had left out in the wet for a bit and noticed rust stains coming from inside the top rails. One end is open, what can I pour inside the rails to stop the rust from draining onto the paint?

 

Eastwood makes a rattle can with a long tube for stopping rust on the inside of boxed frames. I used it on my F-1 pick up bed roll, worked fine

 

Looks like primer? If so, moisture is up there, guessing it has to be sealed somehow. Maybe (again assuming primer), grind down and Por15 maybe?

 

Lol, no it's the top coat, I just stopped mid buff to take a photo of the rust before I buffed it off. Here's a better photo, still mid buff but with the rust gone. With it off the truck I'm thinking I could pour something in there, slosh it around and drain. But maybe the spray with the tube would work. Thinking paint or some kind of rust inhibitor.

Actually once on the truck it won't have much chance to get wet in there but I should do something now...

 

Wow, I wouldn't know, thinking there has to be something you could put on a long flexible rod (like twisted wire) and brush it in there and seal it. Interested to know what you come up with.

 

Sounds crazy, but I've used wd40. Buy it in the gallon can and pour it in there. "Water displacement"

 

Doesn't sound crazy to me. I guess it boils down to something that cures or not. I do have some POR15 though, maybe that would work. Pour it in one, drain out what comes out, pour it into the other..

Thanks for the replies.

 

Couple of problems from what I can see. That spot welded seam doesn't look all that tight. If you pour a phosphoric acid or rust converter its going to stain your new paint. You can treat it but you should be there to wipe up any acid immediately is not to damage the paint. The next thing is to what to put inside that is a water type consistency to get down into that seem. If you do decide to go with the Por 15 use the silver slightly reduced on the 1st coat. 2nd coat full-strength. Using wire with a sponge attached to the end for the rust remover and paint.You could also slosh it in. Be sure to have some wax and grease remover handy if the paint should bleed through the seam you could try taping off the seam. The other alternatives are calcium sulfonate inner panel protector's or something like fluid film. Nice paintwork by the way.

 

The Eastwood stuff with the tube dries hard as a rock after a while. The spray tube has a brass end to disperse it. It will come out through the seam and I used was a dirty diaper green. Maybe do it up side down and wait until it dries to install.