There are products that will convert rust into some kind of stable iron compound. It's mostly used when you can't remove the rust or things are rusted completely through. It will leave a kind of black metal in stead of cast iron. It's the only thing you can do in cases where you are left with rust and little solid iron remains. You can use the simpler methods like in the video with light rust, but if there's lots of pitting, and crumbling surface it tends to need a bit more work before painting. I always go for minimal restorations to keep the original as far as possible, but somethimes it needs the full work.