Yeah haha filler.
The rust has already pitted the metal so if you try levelling it more you'll just decrease the thickness by a lot. Get it treated with kurust (Inside and out as they rust from the inside) to kill the rust, then apply filler in thin layers, sanding as you go. Use primer to see where it needs sanding. Then get it painted, even if it has to be redone later. Primer is porous a lot of the time so just having that wont help, nor will having bare metal, so any paint is better than none.