If they dip it (submerge) that will remove any coating you put on the thing. Then you have clean metal to coat. From there they can use a wand inside the rails. The stuff will still be pulled against the metal inside or outside.
Now I hate coatings. Once you coat the thing it is fixed. Any changes that require wrlding or even drilling in some cases will ruin the coating in that area. Welding to frames with rubberized coatings inside(that hold water) is something I don't recommend.
Instead just work a metal hose inside and soda blast the inside of the frame to fresh metal every 10 years. Then just primer and your good. The thing is steel it will rust. It will rust less with areas to let moisture out vs trapping and hiding rust.