Rustoleum will change the color of the rust, but according to Neil 'don'tlistentomymusic' Young -- Rust Never Sleeps.
My life is a struggle against rust. wire brush and sandpaper or steel wool, then Naval Jelly. Put it on, agitate with a brush until it turns from pink to gray, then wipe most of it off and reapply. Don't let it dry or wash it off.
I get it that there are studs and rafters, but it's not clear if they are for the shed or the greenhouse. But in any case, cubes are inherently unstable. They need triangulation or shear panels. Put in diagonal braces or gusset the corners.
Good luck, we're counting on you.
