Stepside bed rail rust

[Lunnb]

Any suggestions on how to treat rust inside the bed rail tube on a stepside ? I have a 64 long bed stepside with some rust under the rail and inside the tube . Under the rail can be cleaned up pretty well with a wire cup brush on an angle grinder . Inside the rail isn’t too terrible but with the bed off and apart , this is the time to do something . My sandblaster won’t do much past the first few inches on each end . Thought about dragging some sandpaper thru the tubes then treating with ospho, then forcing a rag or sponge with POR15 thru the tube to coat the surface as best I can . Any suggestions are greatly appreciated!