Muriatic acid, that's good stuff. Make sure you have a good set of rubber gloves when working with it. It will strip the oils out of your skin, but boy does it work good.....
Take caution to neutralize any acid (rust converter in a can from the parts store or using an acid solution) completely before painting lots of water, not one little damp rag. Acids can have strange reactions with primers and paint.
can't tell if you are talking about a body panel or the trunk floor, but if the floor, I use Rustbullet http://www.rustbullet.com/ (similar to POR-15 but less $$$).
Just knock down the rust and brush/roll/spray on the rustbullet. My entire floor pan has it inside and out.