Evapo-Rust can be used to remove any rust inside the frame. If you can dismantle the frame, then the individual pieces could be placed into a long tray and the Evapo-Rust poured into it, covering the pieces. The Evapo-Rust would fill inside the tubing, removing the rust. Finding a long tray to fit the longer frame pieces would be the challenge. Maybe a wallpaper tray used for soaking wallpaper to apply. If you can find a way to temporarily seal one end of the tubing, you can pour the Evapo-Rust into the tubing. Let it sit for a day, then rinse it off with a garden hose and let dry in the sunlight.