[Mathias]

Next question, what do I rub the rust off with, sandpaper for metal? If so what grit, don't want to scratch the metal too much...

I'd start with 80 grit to remove most of the rust, then a wire brush (ideally on a small angle grinder) to get the deepest rust pores emptied, then 120, 180, 240, 400 sandpaper to make it smooth. No need to go finer than 400 before painting, especially not under matte/satin paint on a small surface.